Problem adding flange to a sketch with "corners" -> "collision in sheet metal flat pattern"

Hi, 
 I'm very newbie with onshape and trying to model my very first thing. Which is farm equipment fasteners. I can do without the small flanges, but would like to learn how to do them anyways.

This public model https://cad.onshape.com/documents/edc77c3d01ded50a502aee2b/w/c9ffb030211edce5d57e07ad/e/1aa16c100752dbaa15292fa8 shows the problem. So i'm trying to create two small flanges on cut outs. It generates to image correctly, but gives error "collision in sheet metal flat pattern".

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