Basically, it is saying that it can't delete the post because that post's row in the database is still being referenced somewhere else in the database. In this case, it is being referenced by rows in the table where we store votes for the post.

This is the same issue that popped up recently for another user. For whatever reason, sometimes the database misses setting the deletes for post and comment votes to cascade properly. I tried and couldn't replicate the issue in my test environment. rimu is probably making a new database migration that will apply it again and that should fix it.