How to remove imbalance of the brightness by OAG  

You can use the LP tool for that (Light Pollution tool). Add 5 boxes around the image, make sure it's background (by stretching the image to the 30% preset) and calculate, check the result and then add 1-2 small boxes towards any gradient that is left and recalculate. Move boxes towards the gradient, not immediately on top of it.

Hello Vincent

Thank you for your answer.
I will use LP tool for the first time, so I will try variously.

This behavior is due to the fact that FLAT is not properly created?
Is it corrected by the appropriate FLAT even if there is an OAG shadow?

Thank you

If the flats work correctly I think it should be able to yes, I have no experience with this and a OAG, but a flat does correct for the overall illumination profile of the optics. Check if you can see if the flats are indeed applied, in the list below when you calibrate the lights will get "MF" added to the filenames.

Hello Vincent

Christian Sasse's video answered a lot of questions. : Https://www.astropixelprocessor.com/complete-lrgb-tutorial-of-ngc292-the-small-magellanic-cloud-by-christian-sasse-itelescope-net-new-version-app-1-081 /

I also used LP tools like videos and LP tools had a wonderful job.

it is really amazing!

It is isn't it. 🙂 This is where advanced algorithms shine.