Many of the problems people appear to be having with pyenv on M1 are related to it pulling include paths for homebrew that include homebrew libffi, but not passing the library path. For cases like cffi this results in header/library mismatch and you get runtime dynamic linker errors.

Other replies here appear to assume basic compilation issues, but the issues I've seen on various GitHub projects are mostly related to these more insidious C toolchain challenges. They're not M1-specific, but they are manifesting on M1 machines.

Make sure that Xcode is installed, and available, then these instructions work a treat:

https://github.com/pyenv/pyenv/issues/1643#issuecomment-7293...

For installing Python on the M1 Apple Silicon. This will install x86_64 versions, but those will work just fine.

I have this setup running on M1 and Intel. I'm not saying you are having no issues, but I don't think that's the reason.

If you're having trouble installing something with brew on M1, run arch x86_64 brew instead of brew to do a Rosetta installation.

What? I did "pyenv install 3.9.1" on M1 and it worked absolutely fine, it built a arm64 build and everything. Maybe it's because I had Xcode installed?