[Kanegasi]

PutItemInBag wants an InventorySlotId, not a slot in a bag. 20, 21, 22, and 23 are the bag slots. You should also break out of the inner loops once you put the item in a bag.

local ignore,foundspot={[0]={},[1]={},[2]={},[3]={},[4]={},}
for k, v in pairs(EquipmentSlots) do
   PickupInventoryItem(v)
   foundspot = false
   if CursorHasItem() then
      for bag = 4, 0, -1 do
         for slot = 1, GetContainerNumSlots(bag) do
            if not ignore[bag][slot] and not GetContainerItemID(bag, slot) then
               if bag == 0 then
                  PutItemInBackpack()
               else
                  PutItemInBag(bag+19)
               end
               ignore[bag][slot] = true
               foundspot = true
               break
            end
         end
         if foundspot then
            break
         end
      end
   end
end

Edit: Fizzlemizz makes a good point. Based on your wording, it does look like you would rather fill other bags with the unequipped gear before your backpack. I reversed the bag loop so your backpack is searched last.

Edit2: Added CursorHasItem and an ignore table to keep track of spots filled by the code.