iPad Storage is Full
When the iPad storage is full, it is best to go into Storage to see what is filling up the iPad. Go into Settings>General>iPad Storage.
Apple will give suggestions on how to free up space. On top of that, Apple will also give a visual chart of how storage is being used as well as a list of which app is taking up the most space. Depending on the usage, there can be different strategies in freeing up space (deleting apps, videos, pictures, etc).