This message has the following cause and solution:
To provide source code control, Microsoft Access needs to know that the database is being used by only one developer at a time. Therefore, when you first place a database under source code control, Access closes the database and reopens it exclusively, after first asking for confirmation with this message.
Click Yes to have Access close the database, reopen it exclusively, and continue adding the database to source code control. Click No to cancel the operation.