How can have others use the plugin without having op because right now you need op to use the plugin

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

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

I'm pretty new to making servers how can I let players I use that command?

Open the permissions.yml file (in the same folder as the server jar and the plugins directory) and paste this into it:

minepacks:
  default: true
  children:
    backpack.user: true
    backpack.size.6: true