New to SQL: Trying to create a FOREIGN KEY with Sequel Pro on a MAC...yes I know

I'm doing a homework assignment out of Mannino's Database Textbook from Chapter 10. Ive essentially built the database. I had to wait to add the FOREIGN KEY until after I inputted the data due to it not letting me edit any table while the table was under a "parent" so now that i entered the data, going back to add the FOREIGN KEY, i get this error "MySQL said: Cannot add or update a child row: a foreign key constraint fails (hw3.#sql-a9d_8, CONSTRAINT SUPPNOFK1 FOREIGN KEY (SuppNo) REFERENCES Supplier (SuppNo))"