"Multi-device is disabled" error usually appears if you have already had an installation of the Authy app present and you no longer have access to the app (e.g. it was on your previous device).
To resolve the "Multi-device is disabled" issue, you need to reset your account directly with Authy (https://www.authy.com/phones/reset/). Once this process is complete you will be able to install a new Authy app which will not show this error.
Please note that using the Authy multi-device function will enable all your Authy token accounts on the added devices.