If you see the following error when installing PyXLL it is because you are using Python installed from the Windows Store:
ERROR: Invalid Python executable
ERROR: Python installed via the Windows Store cannot be used with PyXLL
Why does this happen?
The Windows Store distribution of Python is a sandboxed application and cannot be used to embed Python into other applications (like Excel) in the way that other more standard Python distributions can.
The internal components of Windows Store apps are protected from being accessed from other applications, and so the PyXLL add-in cannot use the Python DLLs and packages that are installed as part of the Windows Store Python app.
How to Fix it
Install a different distribution of Python, such as the standard distribution from https://www.python.org or Anaconda from https://www.anaconda.com/. Virtually any other CPython distribution will work!