Only me and the admins have the backpack in inventory, but the other members do not, how can it be fixed?

You have to give your players the backpack.use permission. Also you should give them one of the size permissions backpack.size.1, backpack.size.2, backpack.size.3, backpack.size.4, backpack.size.5 or backpack.size.6.
If you do not have a permission plugin please check this link: https://github.com/GeorgH93/Minepacks/wiki/FAQ#how-can-i-use-the-plugin-without-a-permission-plugin