create table sample_test(update_time date);
 /
 create trigger sample_trigger after insert on sample_test ...
 /
 Grant all on sample_test to user_schema;
 /

SQL> create table trig_test (col_1 varchar2(10));

Table created.

SQL> 
SQL> create or replace trigger trig_test_trg before insert on trig_test for each row
  2  begin
  3    null;
  4  end;
  5  /

Trigger created.

SQL> 
SQL> grant execute on trig_test_trg to public;
grant execute on trig_test_trg to public
                 *
ERROR at line 1:
ORA-04042: procedure, function, package, or package body does not exist