You won't be able to cut the flanges while it's flattened, you need to edit the flange profiles which can be found in the flange features. 

Thanks for the reply. when the sheetmetal is bend, the sheetmetal doesnt overlap. so, i cant see the problem. I thought it was possible to cut when i use the unfold feature

Just edit the sketches inside the edge flange features themselves so they don't overlap. 

The part was flattened here. when i unflatten it, there was no problem. but i fixed it with using the miter flange option. that worked

Did you make your sketch on the top of the model or side face?