Why it is not accepted: Jel jsi tam dlouho?
"šeli jste" comes back with the correction that it should be "šel jste". I think that's not right, because "you" could be plural?
You could but does not have to be plural. It can also be a formal you. Formal you for one person we are formal with is ŠEL. Plural you past tense is ŠLI-ŠLY-ŠLA (m-f-n)
I am a native american english speaker I do not understand what the english sentence means. What does it mean to go somewhere "for long"? Must be colloquial