Abag contains 12 slips of paper of the same size. each slip has one number on it, 1-12. describe an impossible event.

There are many impossible events. Here are a few. 
I'm sure you can think of at least 15 more:

-- Pull 5 multiples of 3 from the bag.

-- Pull 3 multiples of 5 from the bag.

-- Pull 7 odd numbers from the bag.

-- Pull 7 even numbers from the bag.

-- Pull two slips from the bag whose sum is more than 21 .

-- Pull three slips from the bag whose sum is more than 33 .

-- Pull two slips from the bag whose sum is less than 3 .

-- Pull three slips from the bag whose sum is less than 6 .

-- Pull two slips from the bag whose product is more than 132 .

-- Pull two slips from the bag whose product is less than 2 .

etc.