.# If enabled the players will be given an item they can interact with to open the backpack.
.# The item may not be removed from their inventory as long as this option is enabled.
Enabled: true

My players are not getting a backpack unless they are OP.

Do I need to have backpack.use permission set?

Minecraft version: 1.15.2

Yes, they need the backpack.use permission. Without the permission they would not be able to use the backpack and therefor there is no reason to give them the item.

I figured it out. Setting the backpack.use permission worked!

Thanks!