The word blocks to choose from for this sentence need an - a' - for the first - a - and an - a- - for the second - a - . An - a - alone has many different meanings, especially if the punctuation marks are left off.
That seems to be a Duolingo glitch. I think the moderators know about it but can't do anything about it.
Yeah you're right. It's been an issue for years: https://forum.duolingo.com/comment/35390549
I used "a-rithist" and Duo marked the hyphen as a Typo. ?