Car won't start after engine rebuild

After a few hiccups during the rebuild, we managed to get the engine in and wired up. Everything was checked and by right's it should have started. When the ignition is on, all of the lights on the dash comes on as normal. Even the red immobiliser light goes out. But when you turn the key to start position, the clocks go black and it won't start. The engine won't start and the starter won't engage. I have scanned it but no codes will come up. A full system check says it cannot communicate with the engine. All fuses are fine and sensors and ground wires are all connected. The battery has also been tesred.Everything is in the right place, something is just b