I wasn't able to get this to build in Visual Studio. For whatever reason it will not build that second namespace.

However, I added IIS Manager to Windows 11, then opened that and pointed the default website at the project top-level folder. I then added an "Application" folder using the tree on the left, and pointed that at the sub-folder.

While it won't build in Visual Studio, it executes perfectly via IIS - both the base site loads and runs, and then using the sub-folder Url configured as above, that also loads and runs.

Not ideal for ongoing development, but this is an older project that just needs a few tweaks, which on this occasion can be done using local IIS as the debugger bypassing Visual Studio altogether.