Im not sure what Tretinoin Cream is so can't answer your question there, but I can tell you that using AHA within the first 4-5 weeks of starting the regimen isn't advised. Your skin will be trying to get used to the BP and the AHA will be adding even more irritation at this early stage.

I have been doing The Regimen for 2 weeks now and is it okay if I used Tretinoin Cream Instead of AHA after Applying Benzoyl Peroxide? Please somebody help me

I have used tretinoin cream prior to the regimen. It works fast when clearing up acne spots for sure. I have read a lot of things online though that it cancels out the BP medication when used together. Not sure why this is.

But I advise you to stick as close to the regimen as you can and not add any other steps until you start seeing good results to your skin.