[Active Directory] Upgrading your first Domain Controller to Windows Server 2019

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.