You created a viewport using our New Viewport tool,and you are now unable to select a viewport within another viewport. Or you simply need to know how to select a viewport within a viewport.
1. Double-click within the larger viewport to activate it.
2. Use the Windows + R keys (Windows keyboard) or Command + R (Mac keyboard) to switch between active viewports until you've selected the one you want.
The viewports select in the order in which they have been created.