I've lost the registration on my Pan 6 on one computer. All computers are Yosemite. How can I re-register?

I need to re-register one of my computers. I use Pan 6 and none of my computers are Sierra. For some reason, the Panorama website won’t accept the serial number that I have for that computer. Does anyone know how to resolve this? I have to say I’m desperate. I use Pan for work and I’m in the middle of a project where I just cannot take time to upgrade.