"Es hat Computer seit einiger Zeit gegeben"
Being an english speaker, I mentally translated this as: "It has Computer since some time given". So, my mind thinks: "gegeben zum Wo? " (given to whom?) How does one translate this sentence?
The German phrase used for "there is/are" is "es gibt" (literally: it gives). "Es hat gegeben" ("it has given") is the perfect tense and means "there has/have been".
Unfortunately, that's where the real problem with this sentence starts. I gathered from some of the other comments that Duolingo wants you to translate this sentence as "There have been computers for some time." However, unlike English, German does not use the perfect tense in since/for constructions when talking about the present. It uses the present tense: "Ich wohne seit einiger Zeit hier" (literally: I live since some time here; meaning: I've been living here for some time) or "Ich lerne seit gestern Französisch" (lit. I study since yesterday French; meaning: I've been studying French since yesterday).
I suppose the German sentence actually means something like "There HAD been computers for some time", although I find the whole perfect construction in German very odd indeed.