Shouldn't it be "доларів" or another plural form if it exists?
Nay, it is fine. When combined with numbers, доларів is used with numerals that end in words for 5,6,7 and larger numbers (in the Nominative).
Why not '€' sometimes?
Only dollars and hryvnias.
How you say 'euro' on Ukrainian?
The hypotext says dollar.. Singular. 21 dollar was wrong though.
The hint gives the translation of the word on its own: "dollar" = "долар," but in the given phrase it translates differently.
"$21" This is spot on correct!
Duolingo does not recognize special characters. If we accept this answer, then "21" will be accepted too (it will not be even marked as a typo), and that would be incorrect.
What?! %$&((*@#%!!!! Hahaha! ;-p