"I cannot be there, that is a private party."
Translation:Nie mogę tam być, to prywatna impreza.
Apart from the typo ("prywanta -> prywatna"), the problem is "ta".
"that" here is not a determiner (as in "that book" or "that fish"), but it serves as the subject of the sentence. And a sentence like "That is Y" will always have "To" as the subject, regardless of the gender of Y.