Anti theft, engine will not start

It sounds like you lost the programming for the security system. Do you have a spare key? I would try this first. Then do a hard reboot on the security system by removing the negative battery cable and hold it to the positive terminal for five minutes this will discharge all system capacitors and let the protocol start over. Reconnect the battery and use the remote to lock and unlock the car.

This guide can help as well.

https://www.2carpros.com/articles/how-to-reset-a-security-system

Please let us know what you find. We are interested to see what it is.

Cheers, Ken