I signed in to my Verizon account and went to Accounts, then Security. There is an area something to the effect Transfer PIN. If you go there, you can get a PIN # to transfer your number to another provider and it will also list your account number. Write those down and you should be all set.
I had the exact same problem. I thought I read something about omitting hyphens in the account number when typing it in for porting. However, if your account number includes a hyphen (aka 'minus sign' "-") you need to include that eg. "1234567xx-xxxxx". . . I'm still working on the porting issue. I got locked out as well and was unable to port. So, I just continued without porting the number. I had success in ordering the phone. But, according to chat service, I'll need to return to the system and port the number after I receive the phone and SIM card. . . . BTW - according the Verizon Wireless you need to request a porting transfer number/pin. To obtain that you need to dial #7678 from the cellphone you want to port. (#PORT, I think) . You'll need that number to port the current number, but haven't done that part yet myself.