The theft system problem comes on the dash and car will not start, but why??

The first thing we need to do is scan the system for codes, you can get a CAN scanner (Controller Area Network) from Amazon.

Here is a video to show you how:

https://youtu.be/u-4syLc-ifQ

and

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

Here is a list of CAN scanners starting at about $25.00 on Amazon:

https://amzn.to/3DBMlc7

Also, can I ask if you have two keys? Try to lock the car using the remote, then let it sit for a few hours and then unlock it, this should reset it. Try it with both keys.

Next you can try a hard reboot of the security system to help reset it which is done by removing the negative side of the battery, with the cable loose from the negative side of the battery, hold it to the positive terminal for 1 minute this will discharge the memory capacitors in the BCM and PCM allowing the system to reboot.

Please let me know what you find.