If your devices are running Android 4.1+ and have already been rooted, please refer to the following steps to complete the authorization.
1. Initiate Remote Control on Desktop client (Win/Mac)、Web Client or AirMirror App.
2. Find the pop-up window on the devices which you want to control, then click GRANT to complete the authorization.
Alternatively, you can open the Root App (like SuperSU) to manually grant the permission for AirDroid.
How to avoid granting permission for AirDroid every time after upgrading/ re-installing AirDroid？
Simply go to Settings of the Root app (like SuperSU) and make the Re-authentication option un-checked.
If you have a problem to root your device, try the non-rooted way here.