Invalid Serial Number

I have been using PE since 2017 with no problems. Suddenly I am asked to login to my Adobe account and then enter a valid serial number, which I did. A message appeared stating the serial number is Invalid. I chatted with an Adobe agent who said I had to go this route as my version is no longer supported by live chat/phone support. The agent did verify that the serial number is the one listed in my account on the Adobe website. The software was purchased from TechSoup (a non-profit software provider.) How to I solve this problem? Thank You.