SketchUp Pro 2018 on a 2017 MacBook Pro Touch Bar:
If you position a section plane, and then select it, It can’t be reposition by drag-moving it.
The section plane appears to be active because I can cut objects if I rotate or move the the objects nearby thru where the section plane was first positioned.
I’ll also add, that If the section plane is initially positioned on the z=0 plane on which all my objects are sitting, the models all disappear. This seems like a problem but I recognize this as normal behavior since in this case all models are above the cutting plane and by default are not visible.