Can Beast Below's "when you play" trigger be put on the chain if you control no other friendly unit to target?

Ruling: No, you cannot put Beast Below's trigger on the chain if you do not control another friendly unit. To put an object on the chain, valid targets must exist for all required targets.

Nuances:

• You can still play Beast Below as a unit even if you cannot put its trigger on the chain
• This targeting requirement is why Salvage received an errata to allow it to be played even when no gear was on the field