当你切换到一个未open的pdb时，或跟你报错相同，但是切换到一个open的pdb，也是无法修改sys密码的，下面为19.13版本测试。
sys/system属于common user，为全局用户，需要在cdb中修改密码。

-- cdb中修改密码
11:00:23 SYS@orcl2 > show con_name;

CON_NAME
------------------------------
CDB$ROOT
11:00:25 SYS@orcl2 > show pdbs;

                                  CON_ID CON_NAME                       OPEN MODE  RESTRICTED
---------------------------------------- ------------------------------ ---------- ----------
                                       2 PDB$SEED                       READ ONLY  NO
                                       3 CZHPDB2                        MOUNTED
                                       4 CZHPDB                         READ WRITE NO
                                       5 TEST                           MOUNTED
11:00:27 SYS@orcl2 > alter user sys identified by "ABCD.";

User altered.

-- mount状态pdb中修改sys密码
11:00:53 SYS@orcl2 > alter session set container=test;

Session altered.

Elapsed: 00:00:00.05
11:01:11 SYS@orcl2 > alter user sys identified by "ABCD.";
alter user sys identified by "ABCD."
*
ERROR at line 1:
ORA-01109: database not open


Elapsed: 00:00:00.08
11:01:14 SYS@orcl2 > alter user system identified by "ABCD.";
alter user system identified by "ABCD."
*
ERROR at line 1:
ORA-01109: database not open

-- open状态pdb修改sys、system密码
11:05:22 SYS@orcl2 > alter session set container=czhpdb;

Session altered.

Elapsed: 00:00:00.29
11:05:29 SYS@orcl2 > alter user sys identified by 'Abcd.';
alter user sys identified by 'Abcd.'
                             *
ERROR at line 1:
ORA-00988: missing or invalid password(s)

11:08:14 SYS@orcl2 > alter user system identified by 'ABCD.';
alter user system identified by 'ABCD.'
                                *
ERROR at line 1:
ORA-00988: missing or invalid password(s)