Revertrisk models require a valid parent revision in order to measure the difference in quality and make a prediction. Therefore, they cannot be applied to (1) the first revision of a page or to (2) a revision that is the only one for a page.
Now the generic error message the model returns is unclear to users, and so they reported bugs (T351021, T347136). To address this, we should provide a more clear error message.
If we want to improve the error message for this case, it would probably require changing the code in the knowledge_integrity module, specifically, the revision.py. The get_parent_revision() will need to raise an error for parentless revisions, and then the model server can catch this error and return users a dedicated error message.
Note that the model server can't differentiate between a parentless revision (first revision), a missing page (page deletion), or missing user information, because in all these cases, the code returns a None back to the model server, which probably should be changed as well.