Translation:I still haven't finished my assignments.
I feel that "I haven't finished my homework yet" should also be correct.
Could just as easily be: I still haven't finished the assignment/s /homework. Adding "my" is not significant and the plural is not necessarily implied.
"I still haven't finished my homework" was rejected but should be accepted
Why in plural? it can be "Assignment" i guess and "Hard" can be used instead of "\Difficult"
should 我還沒有做完功課。 also, "I still haven't finished my homework" or "I still haven't finished my assignments" is better, since 還 represents the meaning of "still/yet" in thiis case.
完成 seems a little bit unnatural in this case... like you're completing some important procedure or task for bureaucratic purposes.
here you want to emphasize you are WORKING on your homework towards COMPLETION. so 做 is the verb associated with doiing homework, and 完 is a marker indicating completion.