Hello, to avoid surprises, is this company version paying or not?
SolidCP is 100% opensource and free to use.
There are no limitations or restrictions in regards of usage.
Compatible with Windows Server 2016?
or only with Windows Server 2012?
Well to you
most easy to check is : https://solidcp.com/tour/supported-software/
it's a relatively up to date list on what is supported.
Windows 2016 is already supported for a long while, windows 2019 support will be added on next release.