Oracle Message PLS-00506 - User Defined Constrained Subtypes are disallowed

ERRORUser Defined Constrained Subtypes are disallowed
CAUSEAn attempt was made to define a constrained subtype, but only unconstrained subtypes are allowed in this release of PL/SQL. For example, the following type definition is illegal: SUBTYPE Acronym IS VARCHAR2(5); -- illegal
ACTIONRemove the illegal type constraint.