This message has the following cause and solution:
When you add a Microsoft Access database to source code control, you must select a source code control project where the files will be added. The source code control provider checks the selected source code control project for any files that already exist that belong to an Access database. If any such files are found, this error occurs.
Do not try to store more than one Access database in a single source code control project. Click an empty project (or one that contains only non-Access files), or create a new project.