Do we say にじゅういちかかん or にじゅういちにちかん for 二十一日間?
I think it should be the former then Google Translate says the latter.
I know Google Translate could well be wrong.
It's the latter, にじゅういちにちかん. 一日 is ついたち, not いちか in the first place. (一日間 is rarely used. but the reading is いちにちかん, not ついたちかん.)
Incidentally, 十二日間 is じゅうににちかん, not じゅうふつかかん. The same goes to over 13 days.