John.vellek providing a summary of the solutions of this issue:
Saved from: https://forums.autodesk.com/t5/autocad-forum/unable-to-delete-move-select-viewport/td-p/4364254
- Verify all layers are on and thawed including the VP layer settings.
- If the Viewport is on the defpoints layer, try renaming defpoints.
- Double-click inside the viewport to make it active.
Maximize the viewport by clicking the + sign in the top left corner.
Change back to paper space and you can now see and select the viewport.
- Draw a rectangle over the viewport
Qselect the viewport
use VPCLIP (viewport is still selected)
Select the rectangle.