"I need to go."
Translation:Мені треба іти.
Should be accepted. In Ukrainian there's a small difference between "іти" and "піти", but it's very intuitive and delicate, so I guess both can be translated as "need to go", if one wants to be flexible.
But actually піти is more like "to start going", "to get going", it's a point in time, not a time period, and іти is just "to go" or "to walk" which happens in a time period