Hello again! after exporting the GDS we obtain this warning:

% feedback find
Feedback #1: CIF error in cell opamp_manuel, layer NSDM: parent and child disagree on CIF

We checked the cell

opamp_manuel

and there is no such problem there. Since it is a warning, do we have to worry about it?

Probably not. Most such errors don't create actual mask problems. It's hard to see such problems because they occur during automatic layer generation. You can do "cif see NSDM" to see where the mask layer is dropped in a flattened design. The feedback area will show an area of NSDM that is created by the child cell. If the feedback area does not appear to create width or spacing errors with respect to the area shown by "cif see NSDM", then it's not a problem. If it looks like a problem, I can explain to you how to make the problem go away.

ok, so in order to address this I have to flatten my design and run the commands you indicate? With

feedback find

a tiny zone is highlighted, as indicated in the figure, there is nothing but nwell and M1 there.

No, you don't need to flatten the design, and that's not advisable. I think I can check on this myself if you tell me what are the parameters (W, L, number fingers, device type) for the device shown there. I am a bit concerned that it is in a position where it would overlap PSDM around the transistor gate, which would not be a legal combination. But if it's just the one error, it's very easy to patch up.