I ran into this repeatedly on my old 32GB iPad. I recently upgraded to 128GB iPad Pro and haven't had a problem since. I believe Fresco requires even more space than it is telling you. When you work on a drawing data is cached on the iPad filling up space. I don't think there is a way to manually remove the cached data with Fresco, but I know there is with Lightroom. If you are also using Lightroom on your iPad you can try clearing the cached data there to make more space available on your iPad. I used to fix the probably by periodically deleting the Fresco app from my iPad and then reinstalling it. All the projects are saved in the cloud so you won't lose anything.