I remember there was a thread about this here but results were few.
Here is what I use in TelloFpv:
13: battery problem / incompatible
14: battery, but don't know exactly what it is
21: tello is tilted
24: IMU problem
29: Motor problem
203: Battery problem again
205: Overheated
206: Tello not yet activated
I'm using the standard Tello app, version 1.4.0.0, firmware v01.04.91.01. This is just a guess, but it might have something to do with "activation." I know you have to activate the drone -- but only because I read about that on the internet -- yet the app has not asked me to "activate" my drone nor do I see any option to manually "activate" the drone.
The app should activate the drone automatically on startup.
If you are on Android you can download the free TelloFpv demo. It will activate the drone on first connection to the drone