I installed Windows Server 2019 recently as described here, but noticed that I couldn’t access Enhanced Session on any of my Hyper-V Gen 2 virtual machines. The setting was greyed out.
Enhanced session gives you a lot of nice features like copy and paste between host and virtual machine, and the ability to resize video, so I really wanted to be able to use it. At first I thought the reason it was missing was due to a missing driver or configuration version of my Gen 2 Hyper-V virtual machines, but it was not.
A quick look at the Hyper-V settings revealed the cause, by default the Enhanced Session Mode ability is not enabled.
Enhanced Session Mode Policy
You need to click on Hyper-V Settings (in the right pane of the Hyper-V manager), then select Enhanced Session Mode Policy, and configure it to Allow enhanced Session Mode as shown here.