Hello, A.P.
Sorry to hear you're having problems w/ your TPMS. During the nine months I've had my 02 SC430, the system has actually worked correctly for only two.
Your car is telling you one of its tires is flat. Maybe one of them actually is, so check the tire pressure.
Other possibilities include one or more of the sensors was damaged during the removal of your old tires and installation of the new ones. I suspect you'll have to go to a Lexus dealer to verify this. If that is your problem, one would think that it's the responsibility of whatever company installed the tires to make it right.
Also during installation, the sensors may have been put in the wrong place-- each sensor has a specific designation (front driver's. rear driver's, etc.). Your sensors may have been put back in the wrong place (front to rear, driver's to passenger, etc.). As an aside, this is but one of the many flaws in the system. Since the car is already expecting the sensors at a specific location, then the car should be able to tell you which tire is flat, even if that means something as crude as four beeps for front driver's, five beeps for front passenger, etc). No, you get the same alarm regardless of where the flat is.
This is also a PITA if you want to rotate your tires, like, you know, a responsible car owner should.
One of your sensors could have been damaged when you hit a pothole or curb (if that happened to you).
Lastly, the battery in one or more sensor could have died. I'm given to understand they have a life of about 5 years. If your car is an 02 and the sensors have not been replaced, then the math works out.
I hope this helps and your problem doesn't cost you anything.
-Joe