"How many hours do you spend on the computer every day?"
Wouldn't that rather mean "For how many hours do you use the computer every day?"? I think there is a subtle difference between putting the duration in between verb and object and putting it behind the object.
following the general rule of time, place, action this should really be 你每天几个小时用电脑？
It looks like 的 is optional: this link shows various options and says that it "might also be used", but doesn't go into detail: https://resources.allsetlearning.com/chinese/grammar/Expressing_duration_with_%22le%22
and this one says that 的 is a part of one of the three major sentence patterns to describe time duration, but under the "key points" section toward the end, says that "it can often be omitted", and has examples at the very end under "pattern 2": https://mandarinfriend.wordpress.com/2015/02/05/expressing-duration-using-time-duration-phrases/
I can tell you that you shouldn't use 每天 (every day) and 都 (both, all) together, but as for the rest of the word order, many of us are in the same boat.
After getting it right previously, I tried adding the measure word after hours and got it marked wrong. The "wait for" questions used the measure word and I'm confused about the distinction of when it can/should be applied.
请你每天用电脑几个小时 I used 请 since there was not a 吗 around to make it a question..but still even with the 请 shouldn't it ve correct?
吗 is only for yes or no questions. You are asking a "how many" question here. Your response was incorrect precisely because you used 请, (even though Duolingo would have still graded you incorrect on account of the 几个小时 placement). Your question is not a question, nor a statement, for that matter. Sounds like: "Please how many hours you use your computer."