I have an item but when I go to an enchantment table to try to enchant it, it appears in the enchanting menu but I can't put any enchantment on it.

how do I allow enchantments to be put on the item?

It uses biped object 51 on the default race.

You need to add keywords to the item to allow it to be enchanted. All vanilla enchantments have a formlist that determines what they can be enchanted on by the keywords in the list. For instance, Fortify Carry Weight can only be put on items with one of these keywords:

ClothingHands

ClothingNecklace

ClothingRing

ArmorGauntlets

ArmorBoots

ClothingFeet

All formlists in the Creation kit starting with the word "Enchantment" hold these lists, which are used in turn by different enchantments.