New here but I think it has something to do with the master branch in gitlab being protected. I tried changing the branch on which Publii commits (and which the gitlab-ci.yml checks) and seems to have resolved it for me.
I just tried pushing to a branch other than master and unfortunately I still get the same error. I also tried unprotecting the master branch, which did not affect the result.
@notedbynoel can you retry pushing to that alternative branch again?