Exit game and restart. But, yeah there are two ways of accessing the engine bay; From above the engine and from below the engine. It's possible the msg just means you need to use the other access method.

I had something like this on another mod. The engine was at the back but I had to select a part that was under the hood at the front of the car and then select the part you wanted at the back, where the engine was. If that makes sense, lol