Can I play Beast Below if I do not have another unit played?

Yes, you can play Beast Below even if you do not control another friendly unit.

According to the RiftJudge FAQ for Beast Below (FAQ #466):

• You can still play Beast Below as a unit even if you cannot put its "When you play" trigger on the chain.
• You cannot put the trigger on the chain if you do not control another friendly unit, because valid targets must exist for all required targets to place an object on the chain.