fatal: The current branch has no upstream branch

Whether your Git remote uses SSH or HTTPS determines which class of problems causes No Upstream Branch, and the fixes are entirely different. Many developers switch between the two protocols without realizing this changes their authentication and configuration requirements. For HTTPS remotes (https://github.com/...), authentication uses username and password — but GitHub, GitLab, and Bitbucket all disabled password authentication in 2021–2022. You now need a Personal Access Token (PAT) as the password, stored in your OS credential store. The Git Credential Manager (GCM) handles this transparently on Windows and macOS, but on Linux it requires additional setup. For SSH remotes (git@github.com:...), authentication requires an SSH key pair: the public key uploaded to your account settings, the private key on disk and loaded into ssh-agent. Run ssh-add -l to see loaded keys. Switch a repository between protocols with git remote set-url origin <new-url>. The git remote -v command shows which protocol your remote is currently using.