[Updated response] I'm sorry you ran into this but, unfortunately, Python 3.6 is in its security-fix-only phase of its life cycles and, as such, we do not provide support on them for new OS versions.  Please see the detailed response to Issue43393 for more information. While the particular error you ran into is easily worked around, the resulting Python will still not execute correctly on macOS 11 Big Sur as there are a number of other run-time issues that would need to be fixed. At the moment, only Python 3.9.2 is fully supported on Big Sur.