One SSH key, one GitHub account
Pushed to a repository I definitely had access to and was told I didn’t. The key was the problem: GitHub only lets a key be registered once across all accounts, and mine was still attached to the other one.
Nothing in the error says that. You get a message about the repository, so you go and check the repository, and everything there is fine.
ssh -T git@github.comThe reply names the account it just authenticated you as. When that’s the wrong name, stop looking at permissions.