There must be ParcelID values in GIS_Data that don't exist in Field_Data so you can't create the foreign key because the data violates the foreign key constraint. You'll have to clean up the data before adding the constraint.

Since ParcelID isn't a primary key in Field_Data, I would have thought you'd get the message,

"There are no primary or candidate keys in the referenced table 'FIELD_DATA' that match the referencing column list in the foreign key 'FK_FIELD_DATA'.