Upgrading Windows Server + Domain Controller to Windows Server 2019 - Fails On "ADPrep.exe"

1. From a command prompt on the server run the following and confirm that the server knows where the FSMO role holder is:

   netdom query FSMO

2. Verify that the server is configured correctly for DNS. If it's the only Domain Controller/DNS Server in the domain then it should be using itself for Primary DNS and using 127.0.0.1 for Secondary DNS.

3. Rerun adprep /forestprep without /forest Dev. I've never seen adprep run with the option of specifying the domain, and it shouldn't be needed in your scenario.

Run NETDOM QUERY FSMO to see the status.

If the Schema role is unavailable or associated with a system no longer available, you may need to seize it.

https://docs.microsoft.com/en-us/troubleshoot/windows-server/identity/transfer-or-seize-fsmo-roles-in-ad-ds