A foreign key column is a column which was specified in the referencing_column of a referential CONSTRAINT definition.
1. The attempt was made to delete a table column which was specified as a referencing_column of a CONSTRAINT definition.
2. The attempt was made to delete the key of a table. The columns of the key were specified implicitly or explicitly as referencing_columns of a CONSTRAINT definition.
Delete the referential CONSTRAINT definition and then the column or key.