You get that error when trying to login using Firebase? The error 400 indicates that something is missing.
I’d recommend erasing and reentering your Firebase info in the thunkable app settings for starters. This step is to ensure you have correct/up to date info. I’d suggest copy/paste from Firebase
Next, ensure in the Firebase console that you have enabled email login as a form of authentication.
Also, I updated the title of the thread to reflect the actual issue