ORA-01652: Extend a tablespace in Oracle 12c, no DBA_DATA_FILES table
EDIT/ !!! Now i see you wrote this:


  Mainly because select * from DBA_DATA_FILES; shows me an error when
  executing, because the table doesn't exist in the database.


In my opinion you just don't have enought rights. You should login as sysdba.

Use this select first, you should see location of datafiles (Change the "EXAMPLE" at the end of this command):

select file_name, AUTOEXTENSIBLE, round(BYTES/1024/1024,1), round(MAXBYTES/1024/1024,1), INCREMENT_BY, INCREMENT_BY/1024  from dba_data_files where tablespace_name='EXAMPLE';


Then add datafile to Tablespace, for example:

alter tablespace EXAMPLE add datafile '/oracle/data/EXAMPLE02.dbf' size 100m autoextend on next 100m maxsize unlimited;