Do I Alter user quota in USERS tablespace? I'm new to Oracle so please bear with me.

After installing Oracle XE 11.2, I created user SALES and granted create session and create table to the user.

Once I logged in with SALES I tried to create a table and I get the error ORA-01950: no privileges on tablespace 'SYSTEM'.

A related question in DBA StackExchange says that I should alter database default tablespace users;. and then assign a higher quota to user, like so: alter user SALES quota 50m on system;

I've already changed the default tablespace to users.

My question is: shouldn't I alter user SALES quota 50m on users; instead of tablespace system?
Yes, users should not write to the system or sysaux tablespaces.  If the user is expected to have data or objects and will be copied to development instances they should have their own tablespace as it makes moving them easier.

Change your file path to what you have and execute this as a privileged user

create tablespace sales_tbs datafile '/home/oracle/databases/ora11/sales.dbf' 
 size 10M autoextend on maxsize 200M extent management 
 local uniform size 64K; 
alter user SALES quota 50m on sales_tbs;


and to make sure all future objects go to the dedicated tablespace for sales

 alter user sales default tablespace sales_tbs temporary tablespace temp;