Ensure your installation media matches your licensing source:
Even after a successful entry, the server may still appear "Unlicensed" until specific services are refreshed. The Primary Fix: Use PowerShell (Exchange Management Shell) exchange 2019 product key invalid
(A domain trust issue can cause licensing validation to fail.) Common Causes of the "Product Key Invalid" Error
This error can occur during the initial setup wizard, via the Exchange Admin Center (EAC), or when running PowerShell commands in the Exchange Management Shell (EMS). This comprehensive guide covers why this issue happens and how to resolve it quickly to ensure your infrastructure remains compliant and fully functional. Common Causes of the "Product Key Invalid" Error via the Exchange Admin Center (EAC)
What did you get the product key from (VLSC, Visual Studio, Retail)?
Solving the "Exchange 2019 Product Key Invalid" Error: A Complete Troubleshooting Guide