Minecraft object can't be further enchanted?

I'm playing Minecraft 1.16. However, e.g. No further enchanting my bow even though there's only one enchantment on it.

I have all the enchantments as a book but in the Anvil they can no longer be put together.

Which enchantments do you want to pack on the bow? Perhaps these enchantments do not exist for the bow.

Do you have enough levels?

Some enchantments can't be combined.

Otherwise I don't know why it doesn't work🙃