"We do not know that girl."
Translation:Nie znamy tamtej dziewczynki.
Wrong case. "tamta" is a Nominative form, used mostly for the subject of the sentence, e.g. "Tamta dziewczyna je chleb" (That girl is eating bread).
"znać" (to know) takes Accusative, and here it's negated, and negated Accusative results in Genitive. Genitive of "tamta dziewczyna" is "tamtej dziewczyny".