When trying to assign specific user or group to a published RDS application inside a collection on Windows Server 2019, we saw an error:

“The security identifier could not be resolved. Ensure that a two-way trust exists for the domain of the selected users. Exception: The network path was not found”

We were only operating inside a single domain so the error did not make sense.

Solution was to run the Server Manager with elevated permissions i.e. right click on Server Manager then select “Run as Administrator”