How to fix error with adding information in Firebase Database?

Question

Android

There is the following function:

if (fullname.isNotEmpty() && password.isNotEmpty()) {
            val email = mEmail
            if (email != null) {
                mAuth.createUserWithEmailAndPassword(email, password)
                    .addOnCompleteListener {
                        if (it.isSuccessful) {
                            val user = makeUser(fullname, email)
                            val reference = mDatabase.child("users").child(it.result!!.user!!.uid)

                            reference.setValue(user).addOnCompleteListener {
                                    if (it.isSuccessful) {
                                        startHomeActivity()
                                    } else {
                                        Log.e(TAG, "Failed to create user profile", it.exception)
                                        showToast("Something wrong happened. Please try again later")
                                    }
                                }
                        } else {
                            Log.e(TAG, "Failed to create user", it.exception)
                            showToast("Something wrong happened. Please try again later")
                        }
                    }
            } else {
                Log.e(TAG, "onRegister: email is null")
                showToast("Please enter email")
                supportFragmentManager.popBackStack()
            }
        } else {
            showToast("Please enter full name and password")
        }

Authentication is successful, but the data in the database cannot be created. Outputs the following: "om.google.firebase.database.DatabaseException: Firebase Database error: Permission denied"
Although, if you go to the firebase console --> Database --> Rules, you will see the following: "You have security rules set that assume open access All users can copy, modify, and delete data in your database."
So what's the problem?