can my oponent use cult the weak if he dont have any unit?

Yes, your opponent can play Cull the Weak even if they control no units.

According to several RiftJudge FAQs, this is a legal play because:

• No Units Required: Cull the Weak does not target, so the choice of which unit to kill happens during the resolution of the spell, not when it is played. Therefore, you do not need to control a unit to finalize the spell onto the chain.
• "Do As Much As You Can": Under Rule 356.3.e.11, if an instruction cannot be fully followed, you perform as much of the effect as possible. If your opponent has no units, they simply do not kill one of their own, but you must still kill one of your units if you have any.
• Not a Cost: Killing a unit is an effect of the spell, not an additional cost to play it. If it were a cost, your opponent would be required to have a unit available to sacrifice to make the card legal to play.