SIM not Valid

The fact that you purchased this phone from Amazon is probably the problem.

Did you buy a Verizon phone? If you want to use Verizon in the US, which is a CDMA carrier, then you need to buy a Verizon phone and if you do this in an Apple store or a Verizon store, then you purchase a Verizon phone and can use it on the CDMA network and yes, the GSM portion of the phone with the sim card slot is always unlocked, so that you can use it for travelling purposes.

If you purchased from Amazon, you could have purchased a GSM phone without knowing this and on entering a Verizon sim card, the phone will lock to the first sim card that you put in it, so it is a sim free phone and not unlocked as Verizon have said.

You need to either return the phone to Amazon and get a full refund and start again - so use the money and go directly to either an Apple store or a Verizon store and purchase the correct phone.

If Apple have told you that it is locked to Verizon in the US, then you need to contact Verizon and tell them this. Tell them that you have spoken to Apple and they have told you the phone is locked to their network and tell them where you purchased the phone and ask them to unlock it for you, unless of course, you have not purchased one of their phones, in which case, there may not be a lot that they can do.

Did you ask the Apple engineer to change the phone's Activation Policy as mentions in the posts here?

https://discussions.apple.com/thread/6763925?answerId=29459376022#29459376022

Thankk for the link Malcolm!