I desperately need help joining a covenant [Spoilers]

I'm so confused on how to join the Warrior Sunlight Covenant.

I've gotten the warrior of sunlight item, and I discovered the altar and have offered it two medals.

But I, for some reason, can't equip or use the item "Warrior of Sunlight" in the covenant section of my inventory.

I've googled and watched a few videos but I am so lost and confused. Please someone explain to me how to use these covenant items! (It won't let me use any of them.)