How can this be a typo when there isn't the option of " a' "?
It's a bug in the word-tiles system I think. There was something about it in the tips pages.
Aye, Duolingo won’t let you skip the apostrophe without the remark about a typo but at the same time it strips all apostrophes from the tiles it gives you to choose from. Unfortunately the Gaelic contributors cannot do anything about it. :(