It appears that because of Curios, Backpacked is incompatible with Elytraslot as they both use the same "Back" slot.

Things I have tried:

• Installed only Backpacked; had default Backpacked Slot.
• Installed Backpacked + Curios; Backpacked had a Curios "Back" Slot
• Installed Backpacked + ElytraSlot + Caleus (required dependency) + Curios (required dependency); Backpacked & ElytraSlot share the same Back slot

Backpacked Only

Backpacked + Curios

Backpacked + Elytraslot and Dependencies

*Did not have picture of it without the other mods, but the same issue of using the same slot persisted.