Hi,
As Microsoft official article states Automatic virtual machine activation, we could activate Windows Server 2019, Windows Server 2016 and Windows Server 2012 guest machine on Windows Server 2019 host machine.
So for troubleshooting the issue, please run command line "slmgr /dlv" as admin on host machine and upload capture here.
Check if any error message occur on guest machine in Event Viewer\Windows logs\application, setup, and system.
Please run "slmgr /ipk ###" as admin, please replace ## with your product key on host machine to activate again. If there is no product key sticker or COA on host machine, try to run "slgmr /ato" as admin. Ater that, please run "slmgr /ipk TNK62-RXVTB-4P47B-2D623-4GF74" as admin on guest machine to activate guest again to check.
Bests,
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