Just looking at this with a customer that just put in a new Windows 2019 server. The machine was purchased with a volume license agreement for Server 2019 Standard. Everything should be super easy, in theory. Problem is, license key from the volume license center doesn't work. We spend a bit of time trying to get this to apply, but no luck.
I have run into a similar situation this morning and not being able activate the VLSC liscense.
Before I decided to call MS and getting frustrated, I was able to activate from an elevated command line.
cscript c:\windows\system32\slmgr.vbs /ipk <product key>
Throwing this out here in case someone comes along this post at a later time.
I've heard that that often works. We had tried that in our case and it had failed.