Raise application error in a stored procedure when inputs contain keywords like 'delete' , 'insert','update'
I would use regex_like:
select column_value
     , case
           when regexp_like(' '||column_value||' ','\s(drop|execute|truncate|create|insert|update|delete|merge)\s', 'i')
               then 'Yes'
       end as illegal
from   table(ora_mining_varchar2_nt
             ('drop table customers', 'just dropping by', 'undroppable'));

COLUMN_VALUE              ILLEGAL
------------------------- -------
drop table customers      Yes
just dropping by          
undroppable               

I've added merge to the list as I assume you are trying to prevent SQL injection. If so, there are still risks even if you filter for keywords. execute isn't a SQL keyword, though. Did you mean call?
Your procedure would become:
create or replace procedure app_error_test
    ( query1 nvarchar2
    , query2 nvarchar2
    , query3 nvarchar2
    , query4 nvarchar2
    , query5 nvarchar2
    , query6 nvarchar2 )
as
    queryconcat nvarchar2(30000) :=
        query1||' '||query2||' '||query3||' '||query4||' '||query5||' '||query6;
begin
    if regexp_like(' ' || queryconcat || ' ', '\s(drop|call|truncate|create|insert|update|delete|merge)\s', 'i')
    then
        raise_application_error(-20032, 'Illegal content in supplied SQL: '||queryconcat);
    end if;
end;