macOS Catalina does not currently create thumbnails for .PSB files in finder.

And if all else fails: http://www.johnrellis.com/photoshop/psbquicklook.htm#1.13

I see. Check this discussion:

https://discussions.apple.com/thread/251767514

If that doesn't work, go into Photoshop > Preferences and look at these File handling settings. Try setting "Maximize PSD & PSB compatibility" to Always. Restart Ps.  In fact might be a good idea to restart your Mac.

You should clear and rebuild your thumbnail cache from Terminal:

Copy and paste this command ar the Terminal and press Enter to run it.

qlmanage -r cache

qlmanage: call reset on cache  < You will see this response.  So let's see how this goes.
  

Catalina (10.15.6 as of now) is the first MacOS to support PSB thumbnails. I can see them in my finder and the open and save dialogs of Photoshop using a 2012 Macbook Pro. It should work.

So why my Mac and not yours?  Look at the slider in the lower right hand corner. You can get it by pressing Cmd / while in the Finder.  Go into thumbnail view and move the slider to the left. Drag the slider to the left until the icons at their smallest. Then move the slider back to the right and see if the thumbnails render.