I have the same issue as stated here: Item of type 'MappingData' can not be saved. Unable to connect to 'net.tcp://localhost:2660' using authentication type 'Windows' and here https://gateway.sdl.com/apex/communityknowledge?articleName=Item-of-type-MappingData-cannot-be-saved. But for me the solutions do not work.
So my impersonation user is NT AUTHORITY\NETWORK SERVICE:
I also have this user in my TRUSTEE table :
I get the same error when I run the Add-TtmMapping command via PowerShell
When I check Event Viewer I also see every 10 seconds the following error, related to the NT AUTHORITY\NETWORK SERVICE account
Is there maybe an issue that NT AUTHORITY\NETWORK SERVICE cannot be used as a valid impersonation user?