If rift herald dies and i choose to play rengar can i play rengar to an battlefield with an enemy unit on it or where i have uints?

No — Rift Herald's Deathknell locks the destination to your base, so neither Rengar can be Ambushed out to a battlefield.

Rift Herald's text is "Play a unit from your hand to your base, ignoring its Energy cost," and that instruction is what's doing the playing (you still pay the unit's Power cost). Ambush only adds options to where a unit may normally be played (Rule 822.1.b–c) — it can't override a destination an effect has already fixed. Same result for Rengar, Pouncing's "I can be played to a battlefield you're attacking": that's a permission, not a replacement for the Deathknell's instruction. This is the same call as the official ruling on Rift Herald + Ambush units (FAQ #12361).

One extra note: Ambush's Reaction timing also doesn't apply here, since you're following the Deathknell trigger's resolution rather than playing him at Reaction timing on the chain — so no enemy-occupied battlefield, and no battlefield where you have units either.