Is this an Asus RT-AC68U? Any otherwise similarly branded clone (eg. a TM branded router) is not supported.
If this is a genuinely supported model, safely remove any USB devices from the GUI, unplug the power from the router, remove the USB devices and wait at least 2 minutes before you power on the router again.
Now, after leaving the router idle for at least 5 or 10 minutes (the internet can be used during this time, just don't access the router GUI) you should be able to successfully flash the firmware you want to use.