Error 30 is communications error. This means that the display can't get a response from the controller, or it doesn't understand the response it gets. That happens when there is a wiring or connection problem (or wrong wiring), or when the display and controller speak different languages (protocols), such as when they are not part of the same brand/model system. Some displays can be bought with firmware for various types of controllers, and if the one you have doens't have compatible firmware for your controller model and version, you may get this error.
it can also happen if the controller isn't being turned on by the display; there's usually a "lock" signal from the display with battery voltage on it, and if that isn't present the controller won't turn on and so can't communicate. If the display / controller wiring isn't correct, this signal could be placed on a low-voltage pin of the controller (like the data lines) and that will destroy the controller, or at least it's ability to communicate. Either of these scenarios means the system won't do anything after you turn it on, no response to your control inputs (pedalling, etc).
Not having a throttle or ebrakes is unlikely to cause this error, though it might cause others if the system is designed to require them.
