Here's how i fixed this problem on my samsung J7
1. Login at verizonwireless.com website
2. Hover over ACCOUNT and click "Privacy Settings" from the drop down menu that appears
3. Links on left side of privacy page will have a section labeled SECURITY, look for "Enhanced Authentication" and click on it
4. Set "Require Enhanced Authentication" to ON and click SAVE CHANGES.
This will send a verification email to your email address. Open that email and click the link provided in the email to verify it.
Back on the enhanced security page, When i refreshed and tried to enable it, i was given a notice that i wasnt allowed to turn the enhanced features on, but i didn't really care about that.
Open your MyVerizon app and try to sign in.... LET ME KNOW IF THIS WORKS FOR YOU!
original problem arose when i switched my plan from PREPAID to POSTPAID (contract) in order to add a 2nd phone/line. Prior to that change, my app worked fine on my Samsung J7 (not an unlocked phone)... hope this helps