Try this:
Go to the bundle:
https://itch.io/my-purchases/bundles
Find the game in question and download it from there. Now, the game should appear in your library.
https://itch.io/my-collections
What do I think happened?
When you buy a bundle, Itch doesn't automatically add all your games to your library, so it's possible it didn't generate the key that proves you own the game. If you download it from the bundle page, it should then be associated with your library, and it should work.
Note: Projects in this bundle are hidden in your library by default until you first access them in order to avoid flooding your library. You can return to this page at any time to access any projects you wish to show in your library.
If this doesn't work, it's best to contact the developer directly in the comments section of that game's page.