Hi zhamari02,
Welcome to the Apple Support Communities!
We understand that you're attempting to purchase additional iCloud Storage but when you attempt to, you're brought back to the Payment & Shipping screen. We also see you've already reviewed Upgrade to iCloud+.
Are any fields on this screen highlighted? Have you verified all of the information is correct?
Do you have access to any other devices you can use to upgrade your iCloud Storage?
Do you experience the same behavior if you restart your device?
If you're using a Wi-Fi connection, does the same thing happen if you switch your internet connection to a cellular connection?
We look forward to your reply.
Take care!