If you just purchased and installed a new Wemo® device, the firmware in it may not be the latest. Depending upon when the Wemo device was manufactured, its firmware may have been released before a Transition Firmware was released.
A Transition Firmware is a firmware that makes a major change from one firmware type to another. If your Wemo device has a firmware version lower than a Transition Firmware, it needs to update to the Transition Firmware version first before upgrading to the latest firmware version.
Transition Firmware version: 2.00.3949.PVT
IMPORTANT: Before updating the firmware of your Wemo device, make sure to update your Wemo App to version 1.13.4.
If you want to check the firmware of your Wemo device, simply launch your Wemo App and tap the Settings icon and go to Settings & About > Hardware Info, then select your Wemo device from the list that comes up and look for Firmware Version.
How to resolve Wemo® firmware update error