I am afraid you have not read (or badly read) the entire description.
First:

 ...to sum their quantity according to each category.

For the lists L and M of example you have to return the string (in Haskell/Clojure/Racket/Prolog a list of pairs):

(A : 20) - (B : 114) - (C : 50) - (W : 0)
where A, B, C, W are the categories, 20 is the sum of the unique book of category A, 114 the sum corresponding to "BKWRK" and "BTSQZ", 50 corresponding to "CDXEF" and 0 to category 'W' since there are no code beginning with W.

Second:

If L or M are empty return string is ""

Do you confound an empty list/array with a count of 0 for some category?

Last:
You are new at CW but before posting "Issue" (which should rather be "Question") 1) look at the top of the page to see how many people passed the kata, 2) read some of the comments before writing yours, 3) note for the future that "Issues" are for problems in the kata.
Good luck at CW and cheers!