UPDATED – I had noticed that the Windows Server 2019 servers were still showing as unlicensed. Re-running slmgr /ipk <Activation key> from the command prompt resolved the issue.
At a client site the other day we introduced two new Windows Server 2019 servers. There was already a KMS host in place however this server did not have the appropriate keys in place to license Windows Server 2019.
After retrieving the Windows Server 2019 KMS keys from Windows Volume Licensing I went to install the keys in VAMT. The first error I receive displayed that this is an incorrect license key, searching the web I came across this Microsoft article.
In our case Windows Server 2016 is supported but you need to be patched to the latest level. The patch to support Windows Server 2019 on a Windows Server 2016 KMS host came out in November. I applied the latest patches and made sure everything was up to date but this article forgot to mention one thing. If you are not using ADBA (Active- Directory Based Activation) you need to make sure that VAMT is at least at the following build 10.1.17763.0 otherwise you will run into problems.
Once I upgraded VAMT then I was able to add my Windows Server 2019 volume license an update license the servers.