Problem with the command "thicken in surfaces"

[corradofiorentini]

Hello everyone.
I apologize for my English.
I drew this surface that comes from a mesh.
At first when I tried to use the "thicken" command it gave me an error.
Then I followed your advice that you had given me in the past: "curvature map analysis" and I found a red area in the lower part of the surface.
I corrected the situation as best I could: the red area disappeared...small points remained (as you can see). I tried again with the "thicken" command.
A strange thing happens to me: if I do +4mm (for example) it works. If I put a number with the "-" sign I get an error.
I attach the file.
Thanks for your help

[davebYYPCU · Accepted solution]

The yellow stitch Icon needs fixing, before the stitch, the surfaces are a lot smaller.

Surface quality will improve with less spline points in the sketches.

Might help...

[corradofiorentini]

Thanks for your help