I'm unsure why you would not be able to vote for anyone. The voting is open to the public as long as you have an itch.io account. It might be set so that participants cannot vote, to prevent them from voting for themselves, that type of thing.
I did just try to rate a project, and yes you are correct. There definitely is some kind of error occurring. I will look into the matter and contact itchio support if necessary. When I try to leave the review it says what you see below despite that criteria being marked:
Please check the information you provided
- no score for Relevance to Theme [How well the Problem and Solution is Communicated]