Firebase data change doesn't work for me

I would like to call a value if it changed in the Firebase database.
This is the data structure:
These are my blocks:

And nothing just appears when a change is made to this Firebase structure.

What am I doing wrong?

Is there a complete guide about this block?

I hope you might have ticked persist. Until it and try

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I had checked it, but never tried the persist for the reason that it always threw me an error.