Answer:
only 2 packages of 1818 cookies
Explanation:
If I want to buy only big packages

Then, i can buy exactly 3636 cookies buying 2 packages of 1818 cookies.
If i want to buy only mall packages

Then, there is no way to buy only small packages and get exactly 3636 cookies.
(with 55 packages i get 55x66=3630 and with 56 packages 56x66=3696)
If i want to buy a combination of both packages i can only buy one big package and the rest in small packages.

Then, there is no way to buy one big and the rest small packages and get exactly 3636 cookies.
(with one big package and 27 small packages we get 1818+27x66=3600 and with one big package and 28 small 1818+28x66=3666)
Then, the only way to get 3636 cookies is with two packages of 1818.