Getting 'ORA00904: invalid identifier' on dynamic tables

I have a script that works with dynamic tables. When executing the below code segment, it gives me error ORA00904: invalid identifier.

IF Database_SYS.Column_Exist (service_tab_, ''KEY_VALUE'') THEN                           
   UPDATE '|| service_tab_ || '
   SET key_ref    = new_key_ref_,                                  
   key_value  = ''Test'',                                  
   rowversion = SYSDATE
   WHERE ROWID    = rec_.ROWID;                                                      
ELSE                                              
   UPDATE '|| service_tab_ || '
   SET key_ref    = new_key_ref_,                               
   rowversion = SYSDATE
   WHERE ROWID    = rec_.ROWID;                           
END IF;

Would be better this one:

BEGIN
    IF Database_SYS.Column_Exist (service_tab_, '''' || KEY_VALUE || '''') THEN
        EXECUTE IMMEDIATE 
            'UPDATE ' || service_tab_ || '
                SET key_ref    = new_key_ref_,
                    key_value  = :val,
                    rowversion = SYSDATE
              WHERE ROWID    = :rid'
        USING 'TEST', rec_.ROWID;
    ELSE
        EXECUTE IMMEDIATE
            'UPDATE ' || service_tab_ || '
                SET key_ref    = new_key_ref_,
                    rowversion = SYSDATE
              WHERE ROWID    = :rid'
        USING rec_.ROWID;
    END IF;
END;

I don't think you need '''' || KEY_VALUE || '''', using just Database_SYS.Column_Exist(service_tab_, KEY_VALUE) should be fine if you have properly coded the function.