The network gateway doesn’t allow for simultaneous firmware updates, so make sure that you are the only one trying to update the firmware.
If an update gets interrupted in the middle of the process (e.g. by force-closing the application), you have to wait at least 20 minutes before you can make a new attempt.
When updates keep failing, try rebooting the network gateway. If the problem persists, contact your installer or the Daikin service department.