Can i say "i eftermiddagen"?
Not in this case. Danish has a different system of temporal prepositions than English does. For times of day, always use om+time+definite suffix.
You can say "i eftermiddag" = this afternoon
Used the wrong article ("en" instead of "et") — "you have a typo". Typed "afrenoon" instead of "afternoon" — wrong answer.
Happens a lot. Might wanna give advice to Duo about this...
Apparently, when in doubt, type 'om'—om means about, if, in, on, and probably the end of the world as well if Danes have anything to say about it.