This is a known and very common issue with the Microsoft Store (UWP) version of Roblox, especially for Robux purchases.
Quick refresh (takes 2 minutes)
This often fixes the popup disappears issue.
- Press Windows + R
- Type wsreset and press Enter
- Let it finish and open the Store
- Restart your PC
- Try buying Robux again
If that did not work:
- Buy from roblox.com in a browser
- Or the Roblox mobile app
- Or use Roblox gift cards