I'm getting the following error message on my MacBook Air when attempting to send a text via Messages+. I have Messages+ installed on my Android phone and have no problems using this phone number on it. I'm on the Verizon network, but via Tracfone. That shouldn't be an issue since Messages+ FAQ says "Who can use Verizon Messages (Message+)?" Anyone with a valid US mobile number can use Verizon Messages (Message+), including non-Verizon customers.
When I put in my husband's number, which is on Verizon Prepaid, it works fine.