Hello saige166.
Thank you for using Apple Support Communities and for sharing these details with us. It sounds like you're unable to upgrade to iCloud+. When you try to do so, you're directed to your billing details. Allow us to assist. To confirm, are you able to make purchases and download apps without any issues from the App Store or iTunes Store? If you're able to download apps and make purchases without any issues, try restarting your iPhone to see if that helps:
Restart your iPhone - Apple Support
You can also try connecting to a different reliable Wi-Fi network or use your cellular network to upgrade to iCloud+. Also, if you have any pending updates available for your iOS device, try installing them. That might help. Just in case, the resources below can guide you through the update process:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
We hope this information is useful. Cheers.