By any chance could it be this issue?
Error: "No licenses found" Activating Server Creator, Explorer, or Viewer Key on Tableau Server 10.5 or Earlier Version
The customer portal shows as a product version 10.5.8. The tableau server currently installed is 10.5.8. I think it may not be that issue.
As a quick sanity check, you could do an install of version 2018.1.x on a standalone environment (virtual machine or physical hardware) and see if the key works. Since 2018.1.x is the first version that recognizes a "Creator Server" key. Just a quick base installation. That is, I wouldn't want/expect you to reveal your current key in the forum.