The vehicle will not start. When you turn the key a Theft light flashes.

Hello,

First you will have to reset the anti-theft system in order for the engine to start.

Try this method:

Put the key into the driver side door lock.
Turn the key once, not all the way to unlock the door.
Leave the key in that position for about a minute.
This gives the anti-theft system to recognize the key.
Then turn the key to the opposite direction.
Remove the key and try to start the engine.
This should reset the anti-theft system.

If you have a key fob, try this:

Hold the panic button on the key fob for 2 seconds.
Close all of the doors and while using the key lock and use the key to unlock the lock the driver's door.
If that doesn't work remove the negative battery cable for 15 seconds and reconnect.

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

Please let me know if you have any questions.
Thank you.