How to add/enable view details for SSRS Report Parameter
The field must be unique, otherwise the reference wouldn't make a good sense. You want to uniquelly identify the record by the value of the parameter. Therefore the table should have a unique index for the field, but it doesn't have to a primary one. Typically it's the ReplacementKey.
Note that you should have decisions like what will the primary key when you designed the table.
You can't create new table relations; table references are used instead. Refer to the example I gave you - CalendarId: