"My eyes hurt because I looked at the computer a lot."
I don't think takusan is the right word here. Takusan means a lot of things, not a lot of time. Maybe it could it be better with よく or 見すぎる
たくさん is valed here. You could say a symilar thing with 見すぎる, but the meaning it a little different. たくさん見て is "to look a lot" and 見すぎる is "to look to much". I do think useing 見すぎる would be more normal and would probably be better to use, but i don't think duoligo expects us to know that grammer at this point.
It's really strange for Duo to break itai into 2 words i - tai. It just doesn't make sense.
It is the subject of the clause, so there shouldn't be a particle.