How to fix DB2 Unsupported functionality: GENERATED BY DEFAULT
Column-organized tables do not support some features, including identity columns. Apparently, in your database the default table organization (the DFT_TABLE_ORG parameter) is set to columns. Either change that or create the row-organized table explicitly:

CREATE TABLE users (
   user_id INT NOT NULL GENERATED BY DEFAULT AS IDENTITY (START WITH 1), 
   user_name VARCHAR(16) NOT NULL, 
   email_address VARCHAR(60), 
   PRIMARY KEY (user_id)
) ORGANIZE BY ROW