It does work, but not as you intended.
If you only want to pick a single item use either :
(defun c:L0 ()
(command "_.chprop" (ssget ":S:E:L") "" "LA" "0" "C" "BYBLOCK" "")
)
or
(defun c:L0 ()
(command "_.chprop" (car (entsel)) "" "LA" "0" "C" "BYBLOCK" "")
)
If you want to select multiple items then you'll probably need to split the (ssget) outside the command (easiest solution) as it requires an enter to end the selection process. Sometimes you can't cut corners
(defun c:L0 ( / ss)
(setq ss (ssget))
(command "_.chprop" ss "" "LA" "0" "C" "BYBLOCK" "")
)