Triggers fire once per insert/update/delete, not once per row. Your line here

select @stokkod=stokkod,@quantity=quantity from deleted

Will fetch only one of the values from the deleted table, while there are as many as rows that you delete.

Maybe something more like this... (rough, untested)

UPDATE stok

SET stokquantity =stokquantity-deleted.quantity

FROM stok inner join deleted on

stok.stokkod = deleted.stokkod