git branch -d <BRANCH>: Fatal - Couldn't look up commit object for HEAD

Make sure you use a capitol D in the command, in this case you would type git branch -D <branch_name>. Note that this will only delete the branch from your local computer

If you are trying to delete a remote branch, type git push origin :<branch_name> (remember to add the colon)

To avoid this confusion you can use

"git push origin --delete branch_name"

this deletes remote branch not local branch.