Run the below query to set up SYSTEM password never expire
SQL> select profile from dba_users where username = 'SYSTEM';
SQL> select profile from dba_users where username = 'SYSTEM';
PROFILE
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DEFAULT
SQL> alter profile DEFAULT limit password_life_time unlimited;
Profile altered.
After doing this...Reset the password of the user again.
SQL> alter user SYSTEM identified by new-password;
User altered.
By doing this,the password will never expire.