See the Github documentation: Dealing with non-fast-forward errors
https://docs.github.com/en/get-started/using-git/dealing-with-non-fast-forward-errors

"You can fix this by fetching and merging the changes made on the remote branch with the changes that you have made locally:"

$ git fetch origin
# Fetches updates made to an online repository
$ git merge origin YOUR_BRANCH_NAME
# Merges updates made online with your local work