This is a common error - it wouldn’t be unusual to use that command 5 or 6 times before it works.
I believe it’s due to a scan being in progress from elsewhere in the system.
I would try the command for 5 mins or so before giving up.
As an alternative, I use a Pi Zero to connect to my Mi Flora sensors - this means my central Hass install doesn’t have to do any Bluetooth LE scanning. It might be useful for you: