SQL & PL/SQL

Hello I have the following statement

CREATE OR REPLACE FUNCTION APDBA.MUNIN_TBS_PCTUSED1
(
  tablespace IN VARCHAR2 
) RETURN NUMBER AS
A NUMBER;
 
BEGIN 
  SELECT round(100*(sum(bytes)/1024/1024)/(sum(maxbytes)/1024/1024),1) INTO A
  FROM dba_data_files
  WHERE tablespace_name = tablespace;
   
  RETURN A;
END MUNIN_TBS_PCTUSED1;
/

The problem is that the user APDBA exists in the database and i am receiving the error ORA-01918: user 'APDBA' does not exist

thanks a lot