You should be using volume media that can be downloaded from the volume center site.
As a work around , mght also try using a kms key as intermediary step.
Dism /online /Set-Edition:ServerStandard /ProductKey:N69G4-B89J2-4G8F4-WWYCC-J464C /AcceptEula
https://video2.skills-academy.com/en-us/windows-server/get-started/kmsclientkeys#windows-server-ltscltsb-versions
then if successful use your own product key below after reboot
slmgr /ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
slmgr /ato
--please don't forget to Accept as answer if the reply is helpful--