This message has the following cause and solution:
You selected the Check Out command, but no files were available for the operation.
To check out a file not yet added to source code control, use the Add Files command to add files. If the file is already in your project, make sure that it is not already checked out.
This error occurs if someone deletes the file or project that you wanted to operate on. Make sure that the project still exists and that you have something checked out.