db2 expired password but cannot reset
https://www.ibm.com/support/knowledgecenter/en/SSEPGG_11.5.0/com.ibm.db2.luw.admin.sec.doc/doc/c0021804.html


  User authentication is completed by a security facility outside the Db2 database system, through an authentication security plug-in module. A default authentication security plug-in module that relies on operating-system-based authentication is included when you install the Db2 database system


If the reset password feature of the CLP is not working for you, you should reset the password directly on the operating-system at the server (or on the security system (e.g. LDAP) that Db2 has been configured to use)