How to schedule function in Oracle?
The error message is very clear, you need to specify a procedure or a valid pl/sql block.
If you really want to truncate the aud$ table from a piece of pl/sql, you will need to use dynamic SQL.

begin
execute immediate 'truncate table sys.aud$';
end;
/


put the code in the action or in the procedure definition of aud_clear_fun, that really needs to be a procedure to work with Oracle Scheduler. The Scheduler is not going to read the returned value ...