I'm having an issue with a loop cut in the image below. The face I'm having the issue is the following:

For other hand my intention is to have the part in blue in the below image totally straight and not curved:

This is the reason I want to loop cut the entire perimeter and move in the Z axis all the edges to make it straight.

I already cleaned up duplicated vertex and the normals are alright but I don't know why is not loop cutting all.