"Ann am mìos."
Translation:In a month.
ann an / ann am means "in" rather than "in the". It's a good idea to memorise ann an/am as if it were one word to avoid confusion with an / am = 'the'.
ann am in mìos a month.
"In the month" would be anns a' mhìos. Duolingo has avoided introducing "in the" in the early part of the course, as the noun then has to go into the dative case, which is not covered in this first part. I imagine you might meet the dative case in the extended version of the course that the moderators are working on now.