ICloud storage

Perhaps you have already activated the 50GB iCloud+ option. When changing to a different iCloud+ storage plan, you should see the current Storage Plan - and available upgrade options.

Settings > [Your Name / AppleID] > iCloud > Manage Storage > Change Storage Plan

Do you have a managed device - perhaps belonging to (or managed by) an Enterprise/Business or Educational establishment? If so, there may be a restrictive management policy applied. In such circumstances, you'll need to seek assistance from the System Administrator that manages your iPad.