Cant commit after starting a merge in Sourcetree

Do a git commit -a once you have resolved the conflicts. This is the last step when you are merging conflicts.

After resolving the conflicts,you should try "git rebase --continue" for the rebase to complete.Post that, commit --amend is allowed.

You can manually delete .git/MERGE_HEAD and Git won't be able to tell that you were just doing a merge. It will let you amend the previous commit with the changes in your index just like normal.

Please read:

Though this would work, it is a hack and not recommended. All is needed here is to let git know the merge is completed git commit -a as per this answer