You are using a smart trainer that is powered by the mains but The Sufferfest app is telling you that your device has a low battery.
Make sure that you remove all other sensor connections related to your trainer other than the trainer itself. See the picture below:
You'll notice that the trainer in the picture is connected as three different devices in the Devices menu:
- FE-C Bike Trainer
- Speed & Cadence
- Bike Power
The FE-C Bike Trainer is the option that you want. Remove the Speed & Cadence and Bike Power connections. Once this is done you should no longer see the low battery warning.