I wrote "i have to know that you love me" and it was wrong. It should be also correct right?
Colloquially, that phrasing would certainly be used. But this is a translation exercise, and "potřebuju" directly translates to "I need." I think it's best to try to stay as close as possible to the Czech or English original, unless a direct translation would be totally weird in the target language... but that's just my opinion.
Why is there a comma in this sentence? I don't think english needs one. What purpose does it serve in czech?
Unlike in English, in Czech all subordinate clauses are separated by commas.