Anti-theft won't prevent you from turning the key in the cylinder,
it will either set off an alarm, IE horn and lights, or not allow the
car to start, or both, (on your Sebring, it's both). If you are
having trouble with the door locks, the tumblers are worn. I had
this problem on a 1998 Sebring that I just bought for my
daughter. I removed and disassembled the driver's door lock
and repaired it myself, but it's a job that takes patience because
of the very small parts involved. If it is the ignition lock cylinder, it
may be either the steering wheel or shifter lock binding. Try
moving the steering wheel back and forth and then the shifter
while attempting to turn the key. If that doesn't do it, then the key
cylinder itself is probably worn out and would need to be
replaced. Good luck!