This is an unpleasant result of the master vs main controversy.

Your local GIT client created a default branch called master (when you initialized the repo with git init), but the remote repository on GitHub has no master – instead the default branch is called main.

Solution A – if you want to name the branch master

Run git push -u origin master instead of git push -u origin main

Or Solution B – if you want to name the branch main

Run git checkout -B main before git push -u origin main