What should I do with this oracle error?

Have you checked to see if you inadvertently added one or more rows to IN_ENTRY before trying to define the constraint? Looks to me like there's some in there whose Product_ID field doesn't correspond to a row in PRODUCT. Then when you try to define the constraint, it's complaining since there's something that doesn't fit the rule. This is why adding constraints after the table creation can be a headache.