This bug keeps resurfacing.

One workaround (which I used) is to Select Existing Environments > Conda > Reload Environments. Once the environments have been listed, Switch to Create New Environments > Conda and then back to `Existing Existing'. Now when any of the listed existing Conda envs are selected, the OK button is active.
Source: https://youtrack.jetbrains.com/issue/PY-77995/Unable-to-add-an-existing-conda-environment-in-2024.3#focus=Comments-27-11355896.0-0

Another workaround (which I have not used) is to set Conda path to some other exe, reloading envs, then changing it to actual conda.exe path and then reloading environments.
Source: https://youtrack.jetbrains.com/issue/PY-77792/No-Conda-enviroment-selected#focus=Comments-27-11175701.0-0