Activate used Verizon iPhone 6 without sim?
If the phone is carrier locked, your best bet is to establish service with Verizon and get it unlocked. It's probably unlocked though so you might have luck putting any carrier's valid SIM into the phone. The SIM doesn't necessarily even need to be active - just that it once was valid and used. 

Basically a Verizon sim will get you past the screen, possibly others as well initially.