As a Free user you can upload to a maximum of 20 files to your Personal Library. You will notice a small lock next your personal files once you reach the limit of 20 files.
Upgrade your account
If you would like to upload more then 20 files to your Personal Library you should first upgrade your subscription. Once upgraded you can upload an unlimited number of files to your Personal Library.
Files that are locked in your personal will be accessible again after you upgraded your subscription.