Hi,
Have you tried the method of KMS key instructed in below post?
Dism /online /Set-Edition:ServerStandard /AcceptEula /ProductKey:N69G4-B89J2-4G8F4-WWYCC-J464C
then after conversion (using your new key);
slmgr /ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
slmgr /ato
Reference links:
KMS client setup keys
https://learn.microsoft.com/en-us/windows-server/get-started/kmsclientkeys
Upgrade Server 2019 Essentials to Standard - specified product key could not be validated
https://social.technet.microsoft.com/Forums/security/en-US/debc3002-3846-4876-a401-fbf2a2a71519/upgrade-server-2019-essentials-to-standard-specified-product-key-could-not-be-validated?forum=winserversetup
Error 1168 – The specified product key could not be validated.
https://doitfixit.com/blog/2019/10/09/error-1168-the-specified-product-key-could-not-be-validated/
Please note: Information posted in the given link is hosted by a third party. Microsoft does not guarantee the accuracy and effectiveness of information.
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Hope this helps and please help to accept as Answer if the response is useful.
Thanks,
Jenny