"The knight is sweating."
This is speculation on my part, as today is my first day of High Valerian. However, I've been learning Swedish for the last two years, and in Swedish there is no difference between "He eats" and "He is eating". (Both are "Han äter.") I suspect that HV treats these the same way. In English, we distinguish between "he is doing something" and "he does something," but that's not universal.
So if you recast this sentence from "The knight is sweating" to "the knight sweats," there's no need for "issa" as a helping verb.
Oh, that makes sence! I am from Sweden and I didn't even think of that. Probably because when I've learned a language at school (I can speak both german and english) we had to think about if it is "he eats" and "he is eating".
Edit: But if it is as you explained, why do they say that you've got the wrong answer if you write "is sleeping" instead of "sleeps"?
Possibly because when one has to translate to English, they need to be specific?
The mouse-over hint on "is sweating" lists both "nages" and "nagios". What is the distinction between these?