A demo repository showing how to use the cffconvert GitHub Action to fail builds when no CITATION.cff file in the Citation File Format is present in a repository.
If you set up the action, so that it doesn't check only on pushes of the ./CITATION.cff path (see configuration in this repository), the action will detect the absence of a CITATION.cff file and fail:
name: cffconvert
on:
push