I had a very similar problem with one of my projects which gave me the same TFS message in Visual Studio 2017. I was also using git. In my case I needed to manually add a comment to the commit before posting the merge. Since I had to manually merge the conflicts, a comment was needed rather than the auto-populated message that takes place on a merge without conflicts.
I’m well aware that commits need comments but if I recall correctly, VS 2015 would auto-populate the comment on a merge, even if you manually fixed any merge issues. Regardless, in VS 2017 you will need to add a commit comment when there are resolved conflicts.