To the right of the steering wheel remove the rectangular plastic panel on the dash. There's a code box in there for the immobiliser (usually yellow).
Disconnect this for a few minutes, reconnect, and then try again........
If you've used the engine ECU from the donor car then you also need to fit the CODE immobiliser box from the same donor car as they are paired units (you will also need the transponder chips from the donor car keys).
You need to fit the engine ECU for your original engine which is already paired with the CODE immobiliser box in your car.
You need to get in touch with Tony at Datareall Services who will be able to "virginise" the CODE box in your car so it can be re-paired with the engine ECU you currently have fitted to your car (see details in this post).