Extrude doesn't create solid

I’m trying to start with Sketchup although I have used AutoCADLT and AutoCAD for a long time.
Simple first question!
I have created a rectangle 150mm x 100mm then added 3mm radius arcs to the corners, erasing the square corners left behind. When I then extrude it by 3mm, the resulting item looks like a solid but Sketchup says it isn’t. Why not? Do I need to join the arc ends to the straight edges? If so, how? TIA

Select all of it and make it a group.

1 Like

Or make it a component.

SU won’t recognise loose geometry as solid - it needs to be a group or (almost always better) a component.

Thanks to both - have now found where to make group or component!

You should hit the Campus to get a handle on the fundamentals.