If you have tried all the above and still see "Exchange 2019 product key invalid," it is time to escalate.
Exchange 2019 is updated via Cumulative Updates. If you attempt to enter a product key on a server that has a partially applied CU or mismatched build number, the licensing service may fail to validate the key. exchange 2019 product key invalid
Troubleshooting Exchange Server 2019 "Product Key Invalid" Errors If you have tried all the above and
Microsoft distributes Exchange Server through various channels. If you downloaded your Exchange Server 2019 installation media from the Volume Licensing Service Center (VLSC) or the Microsoft 365 Admin Center, it expects a Volume License key (such as a MAK or KMS client key). Trying to input a retail key, an MSDN/Visual Studio Subscription key, or an evaluation key into a Volume License build—or vice versa—will frequently trigger validation failures. 3. Cumulative Update (CU) Version Quirks 3. Cumulative Update (CU) Version Quirks