Have 55GB available, but get error message when trying to download movie to add storage

55GB on iCloud or iPad.

You need to clear space on your iPad to make room for the movie download.

iCloud storage is on the internet, not local storage on your iPad.

Go to Settings>General>Usage>Storage. Let all of the apps load in the window. You can see how much storage you have used and how much you have left. You will need about twice the size of the download of free storage in order to be able to download the movie. Also, the iOS will not let you use every bit of available storage on the device.

You can delete content in there if needed, but if you sync with iTunes it's a good idea to transfer purchased into iTunes and backup your iPad before you delete content. If you use iCloud only, backup with iCloud before you delete any content.

How to transfer purchases into your iTunes library

http://support.apple.com/kb/HT1848

How to backup and restore from a backup

http://support.apple.com/kb/HT1766

Having 55GB on iCloud is not going to help you download movies; you need to clear up more space in your iPad.

iCloud space is use mainly for backup.