Translation:We are thirsty, but we do not have water.
Why would it not accept "We hebben dorst, maar we hebben geen water" ??? but in right after it in the other part it accepted " we hebben honger dus we eten". i dont understand when are we using "wij" and "we" exactly?
"have" is used here as main verb, not as an auxiliary verb. Hence, the negated form is "we don't have any water". In the alternative, you could say "we haven't got any water". Since it's an auxiliary verb to 'got', you would use 'haven't'. https://linguapress.com/grammar/have.htm
I still agree with Morna157531: "we are thirsty but we haven't any water" would work fine as a translation in Northern England, for example. I tried "we are thirsty but we have no water" which I think is fine but was also rejected
i had the same question just found this https://forum.duolingo.com/comment/3734337/Grammar-Jij-vs-Je-Zij-vs-Ze-Wij-vs-We
Just out of interest: in Dutch is it obligatory to write a comma between the clauses in sentences like this? (It wouldn't be in English.)
That's not totally correct. There are instances, where a comma is required. It depends on the construct used: https://www.dailywritingtips.com/comma-before-but/