linux-aarch64-musl is the platform one gets when using an Alpine-based Docker image and developing on Apple Silicon, which is a pretty common setup at this point. This set up currently (as of uv 0.4.1) doesn't support installing Python via uv, with the following error:

Searching for Python installations
error: No download found for request: cpython-any-linux-aarch64-musl

For other users encountering this issue, installing Python via app add python3 seems to work for now, although Alpine's packages tend to be fairly minimal, so more dependencies might be needed for larger projects.