I'm going to start with the obvious: Python 2.7 has been dead for 4 years now ([Python]: Sunsetting Python 2). Make sure that you clearly mention the reason for sticking to it, otherwise many users might see it as an XY Problem. I saw in a comment from another question that you are working with [Gwyddion]: Gwyddion which indeed doesn't support Python 3.

Now, since Python 2 is no longer supported, packages, tools, ... don't have to support it (some still might, but most will not - as time goes by).
VirtualEnv that is shipped by PyCharm doesn't. Check [SO]: pycharm does not connect to console with python3.8 (@CristiFati's answer) for a similar situation.

Alternatives:

I. Create the Python 2.7 virtual environment manually and "import" it in PyCharm

I'll be explaining the theoretical process with practical examples. There will be scattered snippets from a Cmd console (starting below):

[cfati@CFATI-5510-0:e:\Work\Dev\StackExchange\StackOverflow\q079004210]> sopr.bat
### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ###

[prompt]> :: === Start ===
[prompt]> dir /b
code00.py

[prompt]>

1. Prerequisites:

   There are multiple variants here, so I'll focus on the one I use most. You'll have to have:

   1. A Python 2.7 instance that will be used as a base for creating the VEnv. I'll be referring to it as (like it was a Nix environment variable) ${PY2}

   2. A Python instance with VirtualEnv ([PyPA.VirtualEnv]: Installation - also check [SO]: Python3.7 venv does not Create Virtual Environment Directory (@CristiFati's answer)) installed. This can be the same as the previous one, but I recommend using a n up to date Python 3 version (${PY3}), as there are some prerequisites as well ([PyPI]: pip, and modern Python versions come with it inside). Might be helpful:

      • [SO]: How to install a package for a specific Python version on Windows 10? (@CristiFati's answer) (important)

      • [SO]: Creation of Virtual environment with python 3.9 version is executed successfully but python version 2.7 or python version 2.2 shows error (@CristiFati's answer)

      • [SO]: Install python wheel file without using pip

2. Create the VEnv

   Step(s) detailed in [SO]: Create Windows Python virtualenv with a specific version of Python (@CristiFati's answer).
   Typically: "${PY3}" -m virtualenv -p "${PY2}" ${VENV2} where ${VENV2} is the location where you want your VEnv to be created at. Activate it afterwards:

   [prompt]> :: === Create VEnv ===
   [prompt]> "c:\Install\pc064\Python\Python\03.10\python.exe" -m virtualenv -p "c:\Install\pc064\Python\Python\02.07\python.exe" ".\py_0207_pc064_venv"
   created virtual environment CPython2.7.18.final.0-64 in 8051ms
     creator CPython2Windows(dest=E:\Work\Dev\StackExchange\StackOverflow\q079004210\py_0207_pc064_venv, clear=False, no_vcs_ignore=False, global=False)
     seeder FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=C:\Users\cfati\AppData\Local\pypa\virtualenv)
       added seed packages: pip==20.3.4, setuptools==44.1.1, wheel==0.37.1
     activators BashActivator,BatchActivator,FishActivator,NushellActivator,PowerShellActivator,PythonActivator
   
   [prompt]>
   (py_0207_pc064_venv) [prompt]> dir /b
   code00.py
   py_0207_pc064_venv
   
   [prompt]>
   [prompt]> ".\py_0207_pc064_venv\Scripts\python.exe"
   Python 2.7.18 (v2.7.18:8d21aa21f2, Apr 20 2020, 13:25:05) [MSC v.1500 64 bit (AMD64)] on win32
   Type "help", "copyright", "credits" or "license" for more information.
   >>> ^Z
   
   
   [prompt]> :: Activate VEnv
   [prompt]> ".\py_0207_pc064_venv\Scripts\activate.bat"
   
   (py_0207_pc064_venv) [prompt]>
   (py_0207_pc064_venv) [prompt]> :: Check for PIP
   (py_0207_pc064_venv) [prompt]> python.exe -m pip > /nul
   e:\Work\Dev\StackExchange\StackOverflow\q079004210\py_0207_pc064_venv\Scripts\python.exe: No module named pip
   (py_0207_pc064_venv) [prompt]>
   (py_0207_pc064_venv) [prompt]> :: @TODO - cfati: Check previous command (error message if PIP not installed, OK otherwise)
   (py_0207_pc064_venv) [prompt]> (py_0207_pc064_venv) [prompt]> echo %ERRORLEVEL%
   (py_0207_pc064_venv) [prompt]> 1
   

3. Install PIP in the VEnv

   This is an optional step that has to be executed only if PIP is not already there (check the @TODO at the end of the last snippet). Usually it's not required, but exemplifying it anyway (you need PIP to be able to install packages in your VEnv).
   Check [AskUbuntu]: How can I find an older version of pip that works with Python 2.7?:

   (py_0207_pc064_venv) [prompt]> :: === Get (v2.7 compatible) PIP (if not already there) ===
   (py_0207_pc064_venv) [prompt]> curl https://bootstrap.pypa.io/pip/2.7/get-pip.py --output ".\get-pip.py"
     % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                    Dload  Upload   Total   Spent    Left  Speed
   100 1863k  100 1863k    0     0  3912k      0 --:--:-- --:--:-- --:--:-- 3956k
   
   (py_0207_pc064_venv) [prompt]>
   (py_0207_pc064_venv) [prompt]> dir /b
   code00.py
   get-pip.py
   py_0207_pc064_venv
   
   (py_0207_pc064_venv) [prompt]>
   (py_0207_pc064_venv) [prompt]> :: Check for PIP
   (py_0207_pc064_venv) [prompt]> python.exe -m pip > /nul
   
   (py_0207_pc064_venv) [prompt]> echo %ERRORLEVEL%
   0
   

   Now, you should have an usable Python 2.7 VEnv.

4. Import the new VEnv in PyCharm

   Steps (PyCharm related ones) are identical to the ones from [SO]: How to install Python using the "embeddable zip file" (@CristiFati's answer)

   After the above steps and PyCharm finishes indexing the files, your new VEnv is ready to use:

   

5. Test the new VEnv from Pycharm

   code00.py:

   #!/usr/bin/env python
   
   import sys
   
   
   def main(*argv):
       print "Py2.7 specifics:", range(7)
   
   
   if __name__ == "__main__":
       print(
           "Python {:s} {:03d}bit on {:s}\n".format(
               " ".join(elem.strip() for elem in sys.version.split("\n")),
               64 if sys.maxsize > 0x100000000 else 32,
               sys.platform,
           )
       )
       rc = main(*sys.argv[1:])
       print("\nDone.\n")
       sys.exit(rc)
   

   Output (PyCharm console):

   E:\Work\Dev\StackExchange\StackOverflow\q079004210\py_0207_pc064_venv\Scripts\python.exe E:\Work\Dev\StackExchange\StackOverflow\q079004210\code00.py 
   Python 2.7.18 (v2.7.18:8d21aa21f2, Apr 20 2020, 13:25:05) [MSC v.1500 64 bit (AMD64)] 064bit on win32
   
   Py2.7 specifics: [0, 1, 2, 3, 4, 5, 6]
   
   Done.
   
   
   Process finished with exit code 0
   

   Also attaching a screenshot:

   

II. Downgrade your PyCharm installation to a version that is still Python 2.7 compatible

[JetBrains]: Other Versions contains a list of them. For example, v2019.3 should be OK, but you'll have to see it for yourself.
Needless to say that it won't have the features an upper-to-date version has.

III. Modify your PyCharm installation to support Python 2.7

Only mentioning this as a purely theoretical variant, I don't even need to start enumerating the reasons why not to do it. I did it once though: [SO]: Run / Debug a Django application's UnitTests from the mouse right click context menu in PyCharm Community Edition? (@CristiFati's answer).