Mods:

• Fabric Loader (v0.16.0)
• Fabric API (v0.100.7+1.21)
• Cloth Config API (v15.0.127+fabric)
• Cardinal Components API (v6.1.0)
• Traveler's Backpack (v1.21-10.0.4)
• Trinkets (v3.10.0) With or without this mod they still were unequipable

1. The backpack can be opened via right-click while selected on hotbar but no key binds work and it cannot be equipped into the chestpiece slot or into the trinkets backpack slot

2. Was in creative mode testing for a new modpack and wasn't able to equip the backpack

3. Being able to equip the backpack into the trinket or at least chestpiece slot

Open backpack from hand, you'll notice button in left bottom corner of inventory, that's how you equip the backpack
Then you can access backpack by B keybind

Ah okay I didn't see that lol, thanks!