If you have installed windows server 2019 windows server 2016 standardevaluation or datacenterevaluation edition to try the new features of the microsoft server platform you can download windows server 2019 free trial or windows server 2016 evaluation here after registering you have 180 days to test it.
Windows server 2016 standard evaluation this edition cannot be upgraded.
If the server is a domain controller you cannot convert it to a retail version.
But on one server it went on going for 16 hours and i had to eventually wipe it out and install fresh with server standard disc on windows server 2012 though it used to be smooth.
During this period all features of windows server 2019 2016 are available to you.
Evaluation versions are great for testing new environments but upgrading to the full version is not as simple as changing the product keys.
Always go with the non evaluation disc on 2016 the best idea.
If the server is running an evaluation version of windows server 2016 standard or windows server 2016 datacenter you can convert it to a retail version as follows.
I tried that recently with windows server 2016 standard evaluation and it returned the message this edition cannot be upgraded.