In temporal contexts fra means in. Examples: fra un mese (in a month) fra poco (in a little while/soon). I guess the idea is that the event occurs between the start and end of the period specified.
The best translation for this sentence 'I'll be free in ten minutes' isn't accepted.
Maybe because duo doesn't like usage of the future tense at this stage?
I read "Sono libro fra dieci minuti" and was totally confused, why for God's sake I should be a book in 10 minutes... Is someone enchanting me???