Yes, donn should be lenited here because an mhairteoil is feminine, singular, and non-genitive. Be sure to report an error whenever you find one, so that it’ll get fixed.
Just a note: dentals doesn't apply with attributive adjectives, which is what donn is here. However, some native speakers don't lenite it.
Since this is an Irish to English translation question, and this course is teaching the Caighdeán Oifigiúil, donn above should be lenited. If this question were an English to Irish translation, then an unlenited donn should be accepted to accommodate those native speakers who wouldn’t lenite it.
As explained in the earlier comments, donn really should be lenited - An mhairteoil dhonn.
There is a rule that says that you don't lenite d, t or s when they occur after d, n, t, l or s (the "dentals" or "dentals/dots" or "DNTLS" rule), and donn comes after a word that ends in l , but this rule doesn't apply to attributive adjectives. That's probably the source of the error in this case.
Is this the actual ‘brown beef’ stewed dish, or can this be any beef that just happens to be brown? (Because it's grilled, painted, rotten, ...)