This has caught me a few times, so I just thought I’d point it out. For the vocabulary word 午前、WaniKani lists and accepts the spelling “a.m.” as an answer. However, for 午後、it only accepts “afternoon” or “PM” with no periods. Writing “p.m.” earns you an error.
Just noting it – if one of those accepts the periods (and it should, that’s the correct AP style), the other should as well for consistency. I keep getting caught on it because I see it and just instantly write “p.m.,” as that’s the most natural way to write it, and hit enter before realizing I’d just screwed it up again even though I am entering the word correctly in English.