Why doesn't "...lecreaza cu tu" work?
Lucrează cu tine would be correct.
Tu is used for subjects. Tine refers to an object.
"tu" and "tine" are two different species of the familiar version of "you" in Romanian. The same way how in Spanish, you would say "contigo" instead of "con tú"
When a pronoun has a preposition before it it's in the accusative case. Tu in accusative case is (preposition) tine. Same goes for eu. It will be (preposition) mine
Duolingo suggests "tata lucrează cu voi". There is no variant "tatal meu..."