Uh oh! What's stopping you from downloading it? Are you getting an error message?
Okay, I did a search on this issue; sadly there doesn't seem to be a definite fix, however this Itch thread (https://itch.io/t/960781/cannot-read-property-build-of-undefined) seems to imply that it may be an issue with the itch desktop app, so if you were using the desktop app try going to https://marinya.itch.io/halloween-party and trying to download it from the website.
Also in the bottom on this Itch thread (https://itch.io/t/960739/cannot-read-property-build-of-undefined-issue), someone says they managed to bypass it just by clicking install a bunch of times, so maybe try that too!