From the gray shading on the line it looks as though you drew the diagonal outside the 4x4 component. You need to open the group (it should be a component) for editing before you draw the line.
When I open the model, I can confirm that. I can highlight the line and the group including the leg is closed. Delete that line.
You should learn to make components, not groups, at least for all elements of your model that are repeated, like the pairs of legs. I almost never use groups - components have many advantages, and only one or two small disadvantages.
And give them meaningful names when you create them. It isn’t helpful to look in the Outliner window and see just repeated and undifferentiated Group followed by Group, and nested Group after Group.
Furthermore it will save you work. Edit one back leg, if they are both components, one a flipped version of the other, and they will both receive the edit.
When I changed the right side back leg to a Component, my SU window crashed, for no obvious reason.
On reopening, I see this in the model and matching selection in Outliner.
Open the leg for editing (double click several times to descend the nested groups to the leg), draw the diagonal. I turned on X-ray view first, to be able to see and pick the back corner for the start of the diagonal.
then push the near triangular face down to the bottom.
Delete the opposite leg group, the move and flip this one to the opposite end to replace it.
