can you play shuriken flip without an enemy unit at a battlefield?

Yes — you can play Shuriken Flip with no enemy unit at a battlefield. The card's damage portion says "up to one enemy unit," and "up to one" includes choosing zero targets (FAQ #3815; Rule 355.8).

What that means in practice:

• You'll skip the "Deal 2" instruction entirely — it has no target, so it simply doesn't execute (Rule 359.3.e.6).
• You still need a friendly unit to move: "move a friendly unit" targets, so with no friendly unit on the board the spell is unplayable (Rule 355.10, similar to the Zenith Blade FAQ).
• This is different from cards like Shakedown that say "Choose an enemy unit" — those require a target and can't be played without one.

So the enemy-unit requirement is optional, but the friendly-unit requirement is not.