This message is displayed if TestComplete is unable to determine a common root folder for all files of the project suite or project you are trying to add to the source control system. This can happen, for example, if your project contains a project item or an element that is located on a different drive than the project or in a shared network folder.
To avoid the problem, you should keep all files of the project or project suite in the subfolders of a common folder. Otherwise, you will have to add the files to the source control system manually.