If you have a Volume License (KMS/MAK) product key but originally installed a Retail version of Office 2021, you cannot simply change the key. The installation bits are different. You must:
Note: Office 2021 uses the same "Office16" folder internally (legacy naming). how to change office 2021 product key
cscript ospp.vbs /unpkey:OLDKEY
cscript ospp.vbs /unpkey:7HXTQ