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

The tests are inconsistent with the requirements and among themselves:

The priority is that the space is as small as possible, so first you must find the smallest spaces that fit, and if there is a tie, place them in the leftmost space.
When placed, the barrels must be positioned from left to right in the available space.

implies that array should be filled frol left to right or from index 0 to index -1(in python)

Some tests follow through with it but some tests don't:
testing for: warehouse = ['', '', '', '0', '', ''], barrels = ['0', '0']
['0', '0', '', '0', '', ''] should equal ['', '', '', '0', '0', '0']

testing for: warehouse = ['', '', '0', '', '', '0', '0', '', '', '', '0', '', '', '', ''], barrels = ['0', '0', '0']
Test Passed

in the first test the issue is that warehouse is expected to be filled frol right to left or from index -1 to 0.
The issue of the other test is that it is complitely fine if array is filled from left to right or from 0 to -1.

Otherwise it's a nice task I had fun solving. Thank you for that.