size() method returns an unsigned integer, so if les is empty then les.size()-1 overflows and becomes a large positive value.

Upd: the difference between your local run and Codeforces is probably the sizes of integer types. On my system sizeof(size_t) = sizeof(long long int) = 8, on Codeforces sizeof(size_t) = 4, sizeof(long long int) = 8.

ill countl = les.size()-1
vector's size can be 0, and it is unsigned int. So count1 will be very big number.

Just write:
ill count1 = int(les.size())-1

while(countl >= 0 and les[countl] >= i)

The problem lies in this line If count is -1 or less than 0 First condition is false but it still evaluates the second condition and returns false

So in that way you are trying to access count[-1]

To avoid this give the second condition inside the loop