Multiple Images/Assets from Super Table?
Its hard to see without the full context of field setup, but as you state, I believe its {% set image = subsection.previewImage.previewImageFile.one() %} that's the issue.

I can see further down you're correctly looking through subsection.previewImage, which are Super Table blocks. Like so:

{% for block in subsection.previewImage %}
   {% set image = block.previewImageFile.one() %}


The issue is that you're trying to call previewImageFile on the Super Table block element query, which isn't going to work.

Maybe you're trying to just look at the first block available and get the image from that? If so, you could do:

{% set image = subsection.previewImage[0].previewImageFile.one() %}