"The tailor is taking the measurements."
Translation:Der Schneider nimmt die Maße.
I'm sure there could be an occasion during which a "tailor" might be taking/making other sorts of measurements, in which case "Messugungen" would be acceptable as an alternate translation for "measurements". (Excuse me - forgot whether "making" or "taking" was used in the original sentence).
Based on the fact that we have been given very little if any context, "Messungen" would still form an acceptable answer as we don't really know what the "tailor" is up to. As long as the answer is grammatically correct, it should be accepted. The "tailor" could be planning to escape from something - all we know is that he's a tailor.
We are made to ASSUME that the measurements he'd be making/taking would be for clothing, rather than in actions. Assumptions can also be incorrect.
Clearly, Duo's already decided which answer is correct, so far as he's concerned, regardless of any other factors.
NB. No offense was intended by this comment.
even if "Masse" can be used for all the measurements of a person (does it always?), this sentence could be referring to multiple people - a team or the cast of a play.
How does that make a difference?
German nouns have a singular form and a plural form -- that's it. There's no "super-duper extra-plural" form.
So measurements are Maße whether you took three measurements from one person or thirty measurements from ten people.
Maßen would be dative plural (and inappropriate here, where you need the accusative case, not dative).
The singular is das Maß.