For Some Administration purpose Oracle need Sys password to be identical so if you change Password On Primary you receive Error :
Check that the primary and standby are using a password file and remote_login_passwordfile is set to SHARED or EXCLUSIVE, and that the SYS password is same in the password files.
returning error ORA-16191
This is error appeared because on run alter user sys identified by password on primary Database which is saved in dictionary tables , how to fix
On Standby Run orapwd command
orapwd file=$ORACLE_HOME/dbs/orapwSID password=newpassword;
Always change sys password on both primary and standby to be the same