perhaps the description is a bit ambiguous indeed,
something like:
"The priority is that the space to insert the new barrels to be as small as possible."

maybe it will get better

because the priority is to find the smallest space, if there is a space that fits but it is larger, the choice will be the smallest space

in this case ['', '', '', '0', '', ''], the smallest space that fits is in indexes 4-5, the space in indexes 0-2 is bigger so they should not be put there

in the case of the warehouse like this: warehouse = ['', '', '0', '0', '', ''], barrels = ['0', '0'], then the barrels must be inserted in position 0-1, because it is leftmost