It appears you are trying to use a very old version of Python 3.6.4 from a python.org installer, and a release candidate at that.  This particular installer version will not run on current macOS systems. Note that Python 3.6 has been in the security-fix-only phase of its life cycle for some years now and will reach end-of-life at the end of this year. The current supported version of Python 3 is now 3.9.5. If at all possible, you should upgrade to it as soon as possible.

If you feel you must use Python 3.6, the last macOS binary installers for it were produced for 3.6.8 in 2018; there are two installers for that release but only the "10.9 and later" installer variant will even launch at all on macOS 11 Big Sur. But keep in mind that there are known issues with trying to run Python 3.6 on macOS 11 (i.e. it is not officially supported) and there have been many bug and security fixes to Python 3 since that installer was built. You should think twice about trying to use it.

https://www.python.org/downloads/
https://www.python.org/dev/peps/pep-0494/