Immobilizer error

Welcome to 2CarPros. What has happened is somewhat common these days. Removing the battery erased the key information from the system. To get it to work again will require either a dealer or automotive locksmith to re-initialize the keys into the system. They will "erase" the current keys, then re-pair the keys you have with the car. The method you are trying will only work if the system still has the key listed as a valid key. In your case it no longer does. With many vehicles these days the best option is to use a memory retainer that is really nothing more than a battery that gets connected to the vehicle to trick it into thinking the main battery is still installed. I built one out of a cheap 12 volt battery and some good leads with clips. Just connect it to the battery post on the fuse box or alternator and a good ground.

Yeah, it's getting to the point that you need to tiptoe through a minefield to work on modern vehicles, and it is getting worse. 2018 started the "Gateway" modules, basically you cannot do anything that requires a scan tool unless you have the factory tool and pay them for the yearly subscription for their info. Change the battery and you need a scan tool to tell the computer that you did it even using a memory saver! Even something like changing the brakes can require a scan tool to retract the electronic parking brakes.

You are welcome and please return with any other automotive questions.