Referential Integrity Q

Is it Possible to change a common relationship between two tables to Force
Referential Integrity after the data has been entered.

I have two tables joined lie this,

Models Table; ModelID (PK), PartsID (LI) Indexed-Yes (No Duplicates)

Parts Table; PID (PK), PartsID (LI) Indexed-Yes (Duplicates OK)

They are linked by PartsID and the primary table is Models Table. There are
no duplicates in PartsID in Models Table, but when I try to Enforce Ref. Int
I get the mesaage "No index found for the referenced field of the primary
table."

Any light shed would be very helpful.
Thanks,
Wally