Thats because you shouldn't. It should go on the s-pipe like everyone has been telling you. I will tell you the reason for it again. With your stock header there is a catalytic converter on it which reduces the airflow before it reaches the O2 sensor in the s-pipe. Now when the header is swapped with one without the catalytic converter now there is no change in the airflow and the sensor detects that and there is your CEL. You place the CEL eliminator so that it tricks it into sensing the change in airflow and eliminate the CEL.
I don't mean to sound like a d1ck about the next comment but if every single person on here has been telling you to do the exact same thing and not what you have done, why havent you listening to anyone? You don't have enough common sense to do the advice you were asking for so I don't even think you should bother modding your car because you will keep coming back here asking questions over and over again without following our advice on how to solve it. So either do what we have told you and solve your issue or just take your header off and sell it and be done with it. Didn't wanna be an a$$hole about it but your issue should have been solved a long time ago if you did what you were told.
Here
https://gmbbodyforum.com/japan-cars/toyota you can know more information about car japan cars.