That's very strange. Try downloading it from the game's page instead of the sale page. That might work.
It was a strange problem! I figured out too late what went wrong on my end and it was the opposite of what you suggested; I kept trying to download it from the game page rather than the sale page and based on later experience, that would have completely solved the problem! Luckily, I was able to claim this game as a community copy thanks to someone's generous donation. Nonetheless, thank you for your timely response and help! I hope to be able to play this game with others sometime, perhaps around 2022's holiday season, or to at least read through it and be able to leave a rating and review. It's the least I could do, after all this kerfuffle!