A similar thing happened to me. I created a branch called something like "Feat/name". I tried to pushed it using :
git push --set-upstream origin Feat/name
I got the same fatal error as you :
fatal: Feat/name cannot be resolved to branch
To solve it I created a new branch as I had very few files impacted. Then I listed my branches to delete the wrong one and it displayed with no cap :
I had used caps before but never on the first caracter. It looks like git doesn't like it...