Problem:
I spent 2.5 hours at a Verizon booth attempting to upgrade my iPhone. The Verizon system would get to a certain step and stop. This happened several times at several different terminals. The salesperson enlisted the help of Verizon's “Back Office” to no avail. The salesperson finally gave up.
Solution:
Several years ago after the Equifax data breach, I froze my credit report and therefore the Verizon purchase system could not proceed.
Hopefully, this information will be helpful to others.