Translation:Max put on tights to do gymnastics.
Elsewhere, DuoLingo accepts the preposition coming after the verb. I thought it would do the same here. Strictly, of course, DuoLingo is right.
Recently, in a "light-hearted" part of the newspaper, the Guardian has highlighted headline errors like this: "Police shoot dead intruders."
Duo is not saying your answer is unacceptable. But it is not included in the computer's incomplete list of accepted answers. You can report "my answer should be accepted". If is reviewed and accepted you'll have the satisfaction of a confirmation. If not you won't know why unfortunately.