In case your Sync button is no longer available on your main Bellabeat app dashboard, it is most probably because your device is no longer paired with your account.
The sync button for Android users is located in the bottom right corner, while for iOS users, it is in the upper left corner.
To confirm that the device is no longer paired, please go to the Settings page of your Bellabeat app and underneath your name, there should be a list of devices that are currently paired to your account. In case the list is empty or missing the device you wish to use, this means your device is not paired and therefore not able to be synced.
To pair your Bellabeat device once again, please got to the Settings page f the Bellabeat app and press the button + Add a new device which then initiates the pairing process and guides you through the pairing of the device.
In case you are having issues pairing your device, please check out this article Pairing issues for some helpful troubleshooting tips.