Hello Sandra Sorte
I have a correction on the above point I mentioned, this is currently not possible in Explore mode of Discovery but you can use Refine mode which allows the specification on a planar as well as curved face.
Please use Refine mode to define the triangles set as output for your case and you will not get that message.
I have also filed an enhancement request for this functionality to be added in Discovery Explore mode.
Apologies for the inconvenience.
Regards.
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