Items representing fansites are items for decoration only. They can not be use in any practical way. The same is with War Backpack . It is only a decoration.
How to check it?
Each backpack existing in Tibia has information about ammount of slots just after you click "look" at it. The most common is "(vol.20)".
There is no info about slots in case of War Backpack, but there is typical for other fansite items description.