A TFS/Visual Studio Workspace represents a mapping between the TFS source control and the folders on your local computer. You can have multiple Workspaces for the same computer, each mapping to different folders so long as those folders do not overlap, as well as for different computers.
In Test Studio, you don't connect a test project to a Workspace. You do connect it to a specific TFS server. With all of this in mind, I'm trying to figure out why you're getting this error message in the first place. I think I need more information to understand what is happening and how to over come it.
1) Using Visual Studio I'd like to see what workspaces you currently have defined and what their mappings are, see attached screen shots for reference.
2) What is the folder path to this new project?
3) Would you take a screen shot of this error message and show me?