I have 2002 deville i changed the battery then theft security came on i cant started

Thanks for choosing FixYa and welcome. The alarm is the reason you can't start the car. You will need the security code. Check your owner manual for this code. Each vehicle is different. You may be able to find it elsewhere, but choose someone that you trust. The dealership can do this for you. They will charge you but you will get the code.

In a lot of cases, you can disarm the anti theft security measures by using your key and physically unlocking the driver's door. Whenever a sudden loss of power occurs, most cars are designed to treat the power loss as an attempt to steal the car. The key in the driver's door lock will usually reset the security measures.

My car makes a click click noise when trying to start

I went to start my 2002 cadillac dts the ignition went click click click and then the message came up that says service theft system-34 tried my other set of keys no result but no message the key fob that i initially tried had a dead battery but the second set of keys were ok. i had disconnected the battery post under the hood and tried opening with the good key fob and left the igniotn in the on position for 10 seconds but the car still would not fire up