Check the attached fix.
You Sculpted the surface bodies into a second solid body for the Combine-Subtract.
Inventor sheet metal does not support multi-body solids - thus the error.
I deleted your Sculpt and deleted the extra surface faces you had. (When Copy Object to get the surface - you only need the outer contact surface).
Now I used Sculpt (or Split) to cut away the excess without creating an solid bodies).
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