There can be several reasons why you can't buy a premium account.
- The payment details are invalid. Head to the app store page where you enter your payment details, and make sure everyhing is correct there. If you use a credit card, make sure it is not expired.
- The funds are insufficient to complete the transaction.
- You have reached your spending limit for that payment method.
- Your device has insufficient storage space.
- If you use iOS, it is possible that your in-app purchases are disabled. To enable this, go to your device's Settings, select "On-screen time", tap on the "Content and Privacy" button, click the "Shopping in iTunes and the App Store" icon, then go over the "In-App Purchases" and tick the "Yes" check box.
Please note that we do not have access to your payment details, so it is difficult for us to tell you exactly why your purchase keeps failing.
The app store and the company that manages your payment method are in a much better position to assist you.