For what reasons can a reduced copy of a DLE image not be created?

Question

DLE

For this, the [thumb] tag is provided in the cms.
First, how does it work? At the office documentation no information. As far as I understand, you just need to set certain dimensions, upon reaching which, a reduced copy will be created. The tag works through add. field.
OK. In the system settings, the restrictions are set:

Result: a link to a thumbnail is generated, but the thumbs folder (where the thumbnails are) is empty.

I've already tried everything.