"I go without you."
Translation:Mi iras sen vi.
Because "sen vi" is a prepositional part of the sentence, in which case you don't use the accusative "-n".
"iri" is intransitive (you don’t "go something"), it doesn’t take an object, hence there is no accusative after it
Why "Mi iras sen vin" not accepted ? I go without /who?/what? -> you, so you is accusative like in "Mi ŝatas vin"