The columns in table <Tablename> do not match an existing enabled primary key or UNIQUE constraint. (Visual Database Tools)
At least one table participating in a relationship must have either a Primary Key or a Unique Constraint. After setting up a Primary Key or a Unique constraint for one of the tables you've selected, you can then define other relationships for that table.
For more information, see How to: Create Unique Constraints (Visual Database Tools) and How to: Create Primary Keys (Visual Database Tools).