"Multi-device is disabled. to enable open Authy on your other device and go to Settings -> Devices"
This message will appear if you have installed Authy on another device and disabled "Allow Multi-Device".
The best practice would be to install Authy on at least two devices and then disable “Allow Multi-Device” after that.
If you are unable to access the other device where Authy is installed, please contact Authy support for additional help:
By email: firstname.lastname@example.org
By Web Form: https://support.authy.com/hc/en-us/requests/new