WARNING: The Python tkinter extension was not compiled and GUI subsystem has been detected. Missing the Tk toolkit?

https://github.com/pyenv/pyenv/wiki#suggested-build-environment

This is a warning so if you don't need TKinter, you can ignore it.

What if I need to use TKinter ?

Then you need to install the Tcl/Tk dependency. MacOS doesn't come with it. The link describes the suggested way to do so. Then reinstall the corresponding CPython version.

Then you need to install the Tcl/Tk dependency. MacOS doesn't come with it. The link describes the suggested way to do so. Then reinstall the corresponding CPython version.

I found that tcl-tk 9.0.2 was already installed so I doesn't need tcl-tk@8

wyattwong➜  Downloads ls -ld /usr/local/opt/tcl*
lrwxr-xr-x  1 wyattwong  admin  22 Jul  3 23:02 /usr/local/opt/tcl-tk -> ../Cellar/tcl-tk/9.0.2
lrwxr-xr-x  1 wyattwong  admin  22 Jul  3 23:02 /usr/local/opt/tcl-tk@9 -> ../Cellar/tcl-tk/9.0.2

But should I use tcl-tk@8 instead as mentioned in #3116 ?

Since ca551b3, Python-Build can detect and pass Homebrew's Tcl/Tk 9 to CPython's build -- but building against it currently fails.

CPython support for Tcl/Tk 9 is not official yet (official MacOS packages are still built against 8), so for this and the above reason, we still recommend using 8 and give it a preference by default.

I couldn't find the exact cause. CPython's build doesn't find the Tk dynamic library, and Homebrew's package, for some reason, doesn't produce libtk9.0.dylib (while it does produce libtcl9.0.dylib) but produces tk9.0 which Clang doesn't find.
So this looks like a 3rd-party problem.