error: Reverting is not possible because you have unmerged files

It is about merge conflicts, from some merge you attempted before. The unmerged files are the files where the merge conflict happened.

As stated in the error message, you should take care of these files and the contained conflicts before you do anything else. One additional option is to do a hard reset - only attempt this if you know what you're doing.

If you want to revert to the previous state of your work do:git merge --abort

In historical order, apply these commands:

git reset --hard <commit-id>
git push -f origin <branch-name>