I paid for 200GB iCloud storage space but on my devices (iPad 16GB) says storage space full?

You need to go to Settings>iCloud>Storage>Manage Storage, click on your device, and see what it is showing for the usage, and available storage. The list of apps and media below will show you what is taking up so much space. If you are using iCloud Photo Library, and you have a lot of photos, that will take up a lot of space. If you are using iCloud Drive and have a lot of files saved to the drive, that can take up a lot of space.

Best of luck,

GB

To confirm, is the error message stating that iCloud storage is full, or the device (iPad) storage?