I have loaded 2D slices using Image Stacks. Before reconstructing 3D volume using these slices, I’d like to use the Scissors effect in Segment Editor to edit only the region shown in the image below (rectangle -yellow)
As the name implies, segment editor edits segments. Your segment is blank, so there is nothing to erase from. First define segment (via threshold perhaps), then you can remove any selection with scissors tool.
If your goal is to make a smaller volume than the original, then use the crop volume module.
Thank you, I just realized that the SegmentEditorExtraEffects plugin has to be installed to use the mask volume after selecting the region using scissors.