Ghouls are a free mint. 1 Soul Pass = 1 Ghoul. Once a Soul Pass has been used for a Ghoul, it cannot be used for a Ghoul again. The total supply of Ghouls is 5,000, and they can be minted until October 1st, 2022.
Check to see if a Soul Pass was already used for the Ghoul mint before you buy the Soul Pass:
Under the Check tab, enter the Soul Pass Token ID and click Check if Soul Pass was claimed for Ghoul. As you can see below, Soul Pass 44 was claimed, so it cannot be used.
Minting the Ghoul
Select the Soul Pass (#495 in below example). This brings up the MetaMask prompt to mint the Ghoul.