Before you can send FIL from the Filecoin Backup App, you must grant permission to cosign transactions in the Fission auth lobby. The transactions page shows you this notification:
Clicking “Request Permissions” redirects you to the Fission auth lobby where you will see a request like:
Selecting “Yes” will grant permission to send up to 1000 FIL before the expiration time indicated by
exp. The expiration time is one hour from when permission is requested.
See the Filecoin Phase 2 Write Up post for technical details on cosigning permissions.