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.
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.