To counter a spell, riposte needs to choose a unit? or it can choose a spell and no unit?

Riposte requires both — a friendly unit and a spell. Its text reads "Choose a friendly unit and a spell," and every choice in that list is a mandatory target. Per Rule 355.8, valid choices must be made for all targets before a spell can go on the chain, so if you control no friendly units, you can't play Riposte even if a spell is on the chain.

The unit target is also what the buff part keys off — you need it to give +[S] equal to the countered spell's Energy cost (and if the unit becomes an illegal target before Riposte resolves, the counter still happens but the buff instruction is ignored).