When you try to perform an upgrade on your first Domain Controller to 2019, you will receive the error
Upon checking the logs. You see the following error
<Message Text="Active Directory on this domain controller does not contain Windows Server 2019 ADPREP /FORESTPREP updates. See https://go.microsoft.com/fwlink/?LinkId=113955." />
In order to solve this, on your domain controller. Launch Command Prompt as Administrator and run the following command
1 cd <ISO/CD Drive>:\Support\adprep 2 adprep /forestprep 3 Type C and press Enter 4 Run adprep /domainprep
Once completed, relaunch the setup and try again.