A significant source of user error is the conflation of two different security layers:
On many networks, you cannot have Call Barring and Call Forwarding active at the same time. Disable forwarding before trying to bar calls. Why Secure Your Call Barring Password? call barring password
Before panicking, try the standard default codes: A significant source of user error is the
Unlike the passcode you use to unlock your screen or the password for your Apple ID or Google account, the Call Barring Password is obscure. It is rarely seen, often never set by the user, and frequently triggers a "Network or SIM password error" that leaves users locked out of their own settings. often never set by the user