You do not have any files checked out from source code control.

This message has the following cause and solution:

• You chose the Add, Checkout, Get, Check In, Undo Check Out command, but after careful checking, no files were available for the operation.

This error sometimes occurs if someone deletes the file that you wanted to operate on (or deletes the whole project, for that matter)- make sure your project is still there. Next, make sure you have something checked out.