Can I play Cull the Weak if I have no units?

Yes, you can play Cull the Weak even if you have no units in play.

According to the official rulings for this card:

• No Units Required: Cull the Weak does not target specific units when it is played. Instead, the choice of which unit to kill happens during the resolution of the spell. Because it doesn't require a target to be finalized on the chain, you can play it regardless of your board state.
• "Do As Much As You Can": In Riftbound, if an instruction cannot be fully followed, you perform as much of the effect as possible (Rule 356.3.e.11). If you have no units, you simply do not kill one, but your opponent must still kill one of theirs if they have any.
• Not a Cost: Killing a unit is part of the spell's effect, not an additional cost to play the card. If it were a cost (like the card Cruel Patron), you would be required to have a unit to sacrifice in order to play it.