An "invalid product key" error for Exchange Server 2019 typically occurs due to version mismatches, incorrect entry methods, or licensing limitations Microsoft Support Primary Causes Version Mismatch
A common misconception is that you need a different product key for Enterprise CAL features. The product key only unlocks either "Exchange Server 2019 Standard Edition" or "Exchange Server 2019 Enterprise Edition." The CALs (Client Access Licenses) are a separate legal licensing matter, not a technical product key.
For automation or remote servers, use the Exchange Management Shell: