Can you cast repulse without a spell target, just with a creature target? Just to get a ''when you play a spell effect''?

No. Repulse requires both targets — a friendly unit at a battlefield and an enemy spell or ability on the chain that chooses that unit and no other friendly unit. The counter instruction is mandatory, not optional ("Counter..." with no "up to" or "may").

Without a qualifying enemy spell/ability on the chain, Repulse fails the legality check at finalization (Rule 358.1) and can't be played at all — even just to trigger "when you play a spell" effects. This matches the ruling for Riposte ("Choose a friendly unit and a spell"): every choice in the list is a mandatory target, and all must be legal before the spell can go on the chain.