That is a cosmetic string stored in branding.dll . To remove it fully, you would need to replace system files or perform an in-place upgrade using a full ISO. It does not affect licensing or functionality. If slmgr /dli says “Licensed,” you are fine.
This error usually happens if your server is currently operating as an Active Directory Domain Controller (DC). Microsoft blocks edition upgrades on live DCs. Windows Server 2016 Standard Iso Not Evaluation
(Note: The Key WC2BQ-8NRM3-FDDYY-2BFGV-KHKQY is the specific GVLK for Windows Server 2016 Standard). That is a cosmetic string stored in branding