iPad storage is full. How do I buy data

Go to:

Settings -> Apple ID (Your Name) -> iCloud -> Manage Storage -> Change Storage Plan

Inside you can select the desired storage space. Remember to set your Photos to upload to iCloud, and make sure the setting in Photos is set for optimization. This means that your iPad only has small versions of your photos lying around until you need to use them. Go to: Settings -> Photos -> Select Optimize iPad Storage

Apps on your iPad are not covered by your iCloud storage space, which means that if it is your apps that take up all your storage space, then you will have to delete some of them that you do not use. The same applies if you have downloaded movies locally, for example in Netflix, or in another app.

If your iPad storage is full, you cannot add storage to the device. Using an iCloud subscription may help, but it depends what is consuming all the data storage on your device.