There is probably a bug in ver. 2021.1.3.205 on webconapps.com. If you click Generate preview in Designer studio and save the process, the preview is available (when clicking on the step names in the Information panel in front end). As soon as I make some changes, e.g. modify a field name or any other modification and save the process again, the preview is not available anymore.
I noticed also that the preview quality (generated image) in front end depends on the designer studio size on the monitor. If I don't have designer studio full screen when I generate the preview, the quality is worse (notice the preview quality on the left hand side of the screen in the attached print screens).