I want to buy a package but why I stuck on the payment page and can't continue?

It could be bugs in your apps. You can follow the steps below:

1. Make sure you are using the latest version of the by.U application
2. Force Close your by.U apps
3. Clear cache your by.U apps
4. Logout then re-login your by.U apps

If it does not work, go ask Nindy on livechat feature to get help! :)