Verizon is perfectly willing to let you use a phone they didn't sell if it is truly compatible with their network (e.g., don't try to activate an iPhone 7 with an Intel modem), and has no encumbrances. I suspect the OP shows a financial obligation on their phone (as someone else has already suggested), and that is why VZ won't activate it.
Just for fun, I tested my no-longer-owned iPhone 6 Plus in the VZ BYOD checker. It was an AT&T designated model, part number MGAU2LL/A, bought directly from Apple. Here's a screenshot. Note how it says both that the phone can be used, and that it is recognized as a non-VZ iPhone.
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