If you’re in a rush just mash those buttons:

vim .git/shallow↵
/<paste offending sha>↵
dd:wq↵

In plain english - remove the sha you’re getting in error message from .git/shallow file. You supposedly have a shallow copy of a repository and in that mode git sends contents of .git/shallow to remote you’re pulling from so that it knows what to send down the internet pipes. And one of those lines in shallow file is referencing an object that has been GCed from the repository.