Your SQL DDL is for a table constraint. For a column constraint a validation rule also needs to be created. This SQL will do that:
CREATE TABLE parts
(
part_id NUMBER GENERATED BY DEFAULT
AS IDENTITY ( ) NOT NULL ,
part_name VARCHAR2(255) NOT NULL ,
buy_price NUMBER(9,2) NULL
);
CREATE UNIQUE INDEX XPKparts ON parts
(part_id ASC);
ALTER TABLE parts
ADD CONSTRAINT XPKparts PRIMARY KEY (part_id);
ALTER TABLE parts
MODIFY buy_price CONSTRAINT Validation_Rule_126_506144642 CHECK (buy_price > 0);
The way I got that to generate was by changing Check Constraint to Alter in my model 2_2021 R1 check constraint example.erwin.