It gives me an error saying that a price must be provided.
Hmm, I don't know then, must be something on itch.io end of things. I would ask the itch.io team directly for more info.
Everything in my control panels and my own testing says that pressing PayPal with 0.00 as the currency entered should be purchasing the game for your account.
For now you can just use the grey link on the pop up "no thanks just take me to the downloads". It won't claim the game, but should let you download it. Drake is one of my older games, so at this point any "update" will most likely be a sequel anyway.