I Cant Purchase Stackable Premium
You can buy through this link: https://wpstackable.com/premium/#pricing-table
If you can't see the payment prompt, you could try to buy in incognito mode/private window browser and see if it works, there might be ad blockers or extensions that might be interfering.
Alternatively, you can try buying Stackable through your WordPress dashboard. Navigate to Stackable in your admin menu, then click on the "Go Premium" tab.