I have been using the free version of Sketchup 17 for a few years.
In January this year I decided to buy a subscription to Sketchup Pro mainly because of layout.
I am using Sketchup for woodworking projects.
I can insert a SketchUp model into layout and the first scene appears in the viewport on the default page.
This is where the confusion arises. I can make a new page, but not a new viewport.
The only way I have managed to make a new viewport is to copy one and paste it into the new page.
You can copy the viewport from the previous page, paste it on the new page and then select a different scene from the Scenes dropdown list in the Sketchup Model panel.
Copying and pasting is one correct way to do this. Another is to go to the Pages menu and duplicate the page. After that you can select the viewport and change the scene. The downside of duplicating the page is that you wind up bringing over dimensions and other stuff you may not want.
BTW, please complete your forum profile with SketchUp version, operating system, and graphics card. That helps us help you.