i bought extra icloud storage why can not download apps?

iCloud storage is online at icloud.com and is accessible to any devices logged into that account for syncing data such as documents, contacts, calendars etc so that they can be available on all devices.

This storage is not actually on your iPad. The storage on an iOS device is hard-wired and cannot be increased. You will have to remove some data from the iPad or buy one with a larger storage.