what should I do
Can you confirm that you have enabled the backround 3d preview? Hover over “available previews” and you’ll see an option pop up on the left side of that section which give you the option of turning on the backround preview. Will look like this:
Have you checked out this page? There are several hints on how to get it working:
Please let me know if that helps.
Thank you so much ,I have solved the problem by your method.