Thanks for reaching out monsserrat,
It sounds like you're trying to assign "Mute" when you use Back Tap. We can help.
The option for "Mute" should be available for you to select as an option. Check out these steps Use Back Tap on your iPhone. If you still don't see the option for "Mute", make sure it isn't already selected for the other Back Tap option - Double Tap/ Triple Tap.
If you continue having difficulty, you can connect with Apple directly to report the behavior using either Get Support or if you're in a region without direct support options, you can also Contact Apple for support and service.
Hope that helps.
Take care.