You need to scroll down to the bottom of this post: https://kaimakigames.itch.io/all-men-are-pigs/devlog/546091/all-men-are-pigs-now...
At the bottom of the post there is a link - click it. It will then compare your Itch email address with the KS back access list. If they are the same it will give you access to the full game.
If you are using a different email address... you'll probably need to message the developer.