It is often said that non-natives use 'это' too much rather than forget it when it should be there. Can you omit it here though?
Yes, you can. In fact, I'd prefer the version without "это".
thanks, I wasn't sure, it said it was wrong. so should be reported.
I just reported it; it's still not accepted though, as of 2018-10-18
Уже седьмой урок я устал this is an absolutely correct translation, but isn't accepted. I reported.
Я просто устала, я не понимаю