Current server configuration is: PDC w/ AD. WIndows Server 2012 R2 64-bit. I have tried mounting the image as a CD, running the install from that as well as copying the entire contents and putting it into a folder on the boot (C:) drive. Each time I try I get the same error message: Windows cannot find the Microsoft Software License Terms. Make sure the installation sources are valid and restart the installation. This is NOT a VM. The hardware is an HPE Proliant DL380 Gen 10. The Server 2019 image works because I tested it by installing it onto a new system. Any suggestions would be greatly appreciated.
It looks like you're using an evaluation media for the upgrade, it is not possible to perform an in-place update from a licensed product to an evaluation.
The recommended and much safer method is to install fresh servers on the side, patch fully and migrate the roles or applications over to them.
If in-place upgrading is the only possibility then you'll need to purchase any interim license (if needed) as well as the end goal license, ask the license provider for the installation media for Windows Server 2019.
In order to perform an in-place upgrade you'll need to run setup.exe from the root of the install media within windows. The media needs to be licensed retail or volume, not evaluation. That being said an in-place upgrade is never recommended and a very risky move. The better option is to build a new one from scratch.
I'd use dcdiag / repadmin tools to verify health correcting all errors found before starting. Then stand up the new 2019, patch it fully, license it, join existing domain, add active directory domain services, promote it also making it a GC (recommended), transfer FSMO roles over (optional), transfer pdc emulator role (optional), use dcdiag / repadmin tools to again verify health, when all is good you can decommission / demote old one.