You are trying to revert a merge commit, and git doesn’t know which parent to revert to. The -m allows us to choose which parent to choose. See the merge commit and note down which parent you want to go to. The parent information can be seen in git log, for example:
commit d02ee0f2179def10277f30c71c5d6f59ded3c595
Merge: dd3a24c 2462a52
and run:
git revert <hash> -m 1
where 1 indicates parent number 1 (dd3a24c).
If you are trying to revert to that commit, do:
git reset --hard <hash>
Understand the difference between git revert and git reset from the docs and decide which one you want. git revert is the safer option, but doesn’t really do what you want. It just reverts the changes of a (set of) commit. git reset makes you move to a particular commit in history, and will rewrite your history.