ORA 01918: user '...' does not exist

Question

I am new to Oracle DB and I am currently working with Oracle Database 21c express edition. After the installation, I created a common user "C##OLUAKIN" so that I don't have to do my work on the SYS or SYSTEM users. At the beginning, things were fine, I created a container database "sqlp" and connected to this database with the new common user "C##OLUAKIN" which I created.

Now, this is the problem: I have created another container database "testatmdb" but I am not able to connect to this database using the C##OLUAKIN user, it returns the error "Status : Failure -Test failed: ORA-01017: invalid username/password; logon denied"

1). I have tried to logon to Sqlplus using the user C##OLUAKIN, I get the same error "ORA-01017: invalid username/password; logon denied"

2). I have tried to change the password of the C##OLUAKIN user by logging on as sysdba and using the alter user command as below;

         alter user C##OLUAKIN identified by Oluakin0823;

but I get error "ORA-01918: user 'C##OLUAKIN' does not exist"

3). I logged on using the SYSTEM user on sql developer and I viewed all users with the query below;

       Select * from all_users;

the common user "C##OLUAKIN" which I created was not found in the list of users.

The puzzling thing however is that I am able to connect to the other container database "sqlp" using this C##OLUAKIN user which the system claims does not exist. In fact, while logged on to this database, if I query to check the current user, by running

     select * from user_users;

the query returns C##OLUAKIN as the current user as seen below

So, I am confused. *Why am I not able to use the C##OLUAKIN user being a common user to connect to another container database? *Why is the C##OLUAKIN user missing in the list of all_users? *Why is the user reported as not existing even though I am able to connect to a database through the same user?

Please help.