Hello and welcome to Apple Support Communities hexa997,
Thank you for contacting us. You have come to the right place and we are happy to assist you. If we understand correctly, you have updated your payment method but still unable to make mobile purchase. We can understand why you’d reach out for a solution. To start, have you contacted your carrier ? Make sure you have the most current Operating System on your device (Settings > General > Software Update > should be 14.7) Are you using a private Wi-Fi that uses a password?
If you can get back with us with more details, and the answers to above questions, we will be better able to get this resolved for you.
Thank you for contacting Apple Support Communities.
Best regards.