You have some orphaned data that you'll need to cleanup before adding the constraint.

SELECT RIDER_NO

FROM TRIP_DETAILS

WHERE RIDER_NO NOT IN (SELECT RIDER_NO

FROM RIDERS);

Or you can do this incrementally by adding the constraint WITH NOCHECK to in effect prevent the creation of new orphaned rows but tolerate the existing orphans, then cleaning up the bad data as time allows, and finally once all orphans are removed or resolved going back and CHECKing the constraint:

-- 1. add constraint but do not check existing rows

ALTER TABLE TRIP_DETAILS WITH NOCHECK

ADD CONSTRAINT fk_TRIP_DETAILS_RIDER_NO

FOREIGN KEY (RIDER_NO)

REFERENCES RIDERS(RIDER_NO);

-- 2. cleanup orphans as time allows...

-- 3. check the constraint to let the database engine record that it can rely on the existing data conforming to the constraint

ALTER TABLE TRIP_DETAILS CHECK CONSTRAINT fk_TRIP_DETAILS_RIDER_NO;