It’s quite simple: f6ff240dbf47234249a68b34c8a98bb11237aa7 doesn’t exist.
I just realize you mean 3f6ff240dbf47234249a68b34c8a98bb11237aa7 (Note the missing 3 at the beginning) Just go to the commit list and type Ctrl+f f6ff.
If that is someone you know, you should tell them to always branch from master and commit their custom changes there (for every change), instead of committing directly into master. Then he doesn’t need to rebase and force-push the changes from upstream (you) and you only have to merge the specific story-branches.