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.