How does "not something we can merge" arise?

I got a same error message. In my case, the reason is I use '(' and ')' in branch name.

I get the error if I use the wrong quotes (HEX UTF-8 bytes E2 80 9C and HEX UTF-8 bytes E2 80 9D) instead of the simple quote (HEX UTF-8 bytes 22) to enclose the commit message, see below.

Master@HP-ENVY-L MINGW64 ~/desktop/learning/git-practice/p_2 (master)
$ git merge feature-branch -m “Merge feature-branch into master”
merge: into - not something we can merge

Master@HP-ENVY-L MINGW64 ~/desktop/learning/git-practice/p_2 (master)
$ git merge feature-branch -m "Merge feature-branch into master"
Already up-to-date.

It happened because I copied and pasted the command from instructions instead of typing it in myself. The message git provides is rather unspecific.

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.