Fix Paddle Checkout Error -- 2

I'm integrating Paddle into the backend of my digital-goods store on Replit and keep running into a blocking message: “It says it's a checkout erro.” Everything else is wired up, but the checkout refuses to complete and I can’t see what I’m missing... (Budget: $10 - $30 USD, Jobs: API Integration, Backend Development, HTML, MySQL, Payment Processing, PHP, Replit, Software Architecture)