Crazy Czech numbers. BYLI would be correct if there were 2, 3 or 4 men. Anything 5 and up (including descriptive amounts like few, many) start behaving weird. This is true across all genders. More about this weird phenomena under "numbers 2" tips and notes. https://www.duolingo.com/skill/cs/Numbers-2
But... this is not the English course, is it? Please, by all means, feel free to correct me, but the purpose of the course is learning Czech? Formulating it as "there were here twenty one men" proves the student knows the meaning and the translation, beyond any reasonable doubt. Granted, the English is a bit strained here, but that is rather outside the scope of the course, isn't it?
Many students take this as a reverse course (for learning English), which is a practice common to many courses on Duolingo. If that were not enough to convince you: All the accepted translations have to be entered by hand, and it is enough effort to add the correct ones. Asking the volunteers to spend their time on wrong translations just for convenience of some users is a bit too much.