Adobe is the only one who can address and correct activation matters. We are not Adobe. Just user to user.
Please contact Adobe via its Adobe Chat by clicking on the following link. When it opens, click on "Still Need Help? Contact us." to bring up the Adobe Chat to discuss "Serial Number Issues or Activating My Product"
There was a Mac serial number invalid issue a while back. I will see if I can find it. But, Adobe Chat should be your target for the resolution here.
Please let us know the outcome.
I came across this older thread Premiere Elements 12 Mac related....
but according to the following later thread, the Premiere Elements 12 Mac serial number issue was taken care of on the Adobe side.