I bought the game while it was on-sale (50% off). For some reason, I can't access the latest version. Does this mean I have to pay for the game again or is there something wrong on my end? (I'm playing on Windows btw.)
No. You don't have to pay again. It's in early access now. It was received by those players who spent $10. After 1-2 weeks the game will be available for $5.