What is the USB app?
Do the textbox test projects work for you?
It would really help if you provided a screenshot of your relevant blocks, so we can see what you are trying to do, and where the problem may be.
To get an image of your blocks, right click in the Blocks Editor and select "Download Blocks as Image". You might want to use an image editor to crop etc. if required. Then post it here in the community.
How to avoid runtime errors with the companion app
After importing an extension, please restart the companion app.
If you like to use an extension on a different screen, in Screen1 additionally drag the extension into the working area.
While starting the companion app, all necessary assets and also the imported extensions of your project will be copied to your device. So in case the no such class error shows up, the extension code to run the extension is not available on your device. Without that code the companion app is not able to execute methods from the extension
When I connect through with the USB method, the companion app is not running, but starts itself when I connect and with the error, I would shut down the app.
What I did was reinstall the companion app on my phone and it now works, both your Test app and in my app, with no errors.