There's a very poor description of the condition in the kata!!!
I'll add a condition that will help you solve the problem.
You need to check that it does not return null variants.
For example, you have stock_list([[“ABART 20”, “CDXEF 50”, “DKWRK 25”, “BTSQZ 89”, “DRTYM 60”], [“W”, “H”]))

Your program will return the value "(W : 0) - (H : 0)", but since all values here are NULL, the program MUST RETURN an empty string ""