Hello everybody, I am using the 3DExperience platform. In the Generative Shape Design of CATIA, I have my structure composed of different parts joined together in 3D and I am trying to transform that into an assembly of 2D parts using Transform --> Mid-surfaces for later do a simulation of shell modelling.

However, I am used to do mid-surfaces in automatic mode so small gaps between parts appear because the contact was done in 3D and now we are going to 2D. I then try to do mid-surfaces using "faces to shift" but when I am choosing my supports, I get the same error message over and over (see attached) whereas I am doing what it tells me to do in that message. i was wondering if anybody already had that problem and solved it?

3DEXPERIENCE PLATFORM ​​​​​​​CATIA ​​​​​​​Edu ​​​​​​​

Thank you for your help

PS: sorry, thye error is in French, translation in English:

"Invalid selection.
The selected item must be a face from the input support.
Select a side from the input support."

CATIA ​​​​​​​Technical Domain usge ​​​​​​​Generative shape Design ​​​​​​​Edu ​​​​​​​