From which FlexiCapture 12 versions Windows Server 2016 and 2019 are supported?
ABBYY FlexiCapture 12 Release 2 Servers support installation on Windows Server 2016.
But if you need to install FlexiCapture Stations on Windows Server as well, then you need the "Desktop Experience" feature to be installed. You can see in the System Requirements, that "Desktop Experience" is only needed for stations. FlexiCapture stations are desktop applications, and the Desktop Experience feature makes it possible to install and work with applications on your server because in this case all the features for system-user interactions are installed.
Please note that on Windows Server 2016, the "Desktop Experience" feature can only be selected at the moment of Windows installation. Please see the information from the Microsoft website:
Unlike some previous releases of Windows Server, you cannot convert between Server Core and Server with Desktop Experience after installation. If you install Server with Desktop Experience and later decide to use Server Core, you should do a fresh installation.
FlexiCapture 12 started to support Windows Server 2019 from build 18.104.22.16802.