Unable to upgrade iCloud storage

Hello kieu-my,

Thanks for reaching out to Apple Support Communities.

What happens when you try to upgrade your storage plan using the following: Upgrade to iCloud+.

How to upgrade on your iPhone, iPad, or iPod touch

Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage.

Tap Buy More Storage or Change Storage Plan.

Choose a plan and follow the onscreen instructions.

If you're on your iPhone, iPad, or iPod touch with iOS 14 or iPadOS 14 or later, tap here to upgrade to iCloud+. 

Are you receiving any error messages? If so, what does the error message state? Don't provide any personal information.

Make sure that you have a valid payment method and that your information is up to date:

Change, add, or remove Apple ID payment methods - Apple Support

Payment methods that you can use with your Apple ID - Apple Support

Take care.