or At last, it's the weekend! or It is, at last, the weekend or even The weekend, it is, at last! ;)
Actually, The weekend, at last! or At last, the weekend! would also make sense to English listeners. Duo doesn't have smart enough algorithms to auto-generate every possibility.
I thought here is 'här' in swedish. what word means Here in the sentence above?
Because that is not correct English; "finally" is not a noun. The "det" here means "it," not "the," by the way. "Helg" is common gender anyway (an -en word), and in Swedish, we always use the suffixed definite article even when we use the separate one ("den roliga helgen"), and there's no need to use the separate one at all unless there is an adjective or something ("Äntligen har helgen kommit."). So this sentence literally says "Finally is it weekend," which, of course, wouldn't work in English without a little tweaking.
I read through the comments, and I agree with the thread by @person222222 below , Äntligen är det helg!" would translate better to "Finally, it is the weekend!". Correct me if I'm wrong but wouldn't the English translation of, "The weekend is finally here!" be more closely matched in Swedish to, "Helgen är äntligen här!"? Sorry, just a Swedish beginner, so what do I know? :-)