Best reported from the quiz through report it button. Your answer and the question can then be checked directly. However, if you wrote 'en retarde' as above, that may be the reason it has been marked incorrect. 'En retard' is a fixed, invariable adverbial expression - no agreement.

https://www.wordreference.com/fren/en%20retard

En retard

Bonjour June,

https://progress.lawlessfrench.com/questions/view/expressing-lateness

See Aurélie's reply in this link above.

Je suis en retard de dix minutes is confirmed correct there.

But you have written "retarde" not retard, so this may be why you have been marked wrong -- a simple spelling error.

Bonne journée

Jim