What does this error mean: "Error from server: Slug is not unique. The field: "slug" errored because "Slug is not unique"?

The error message is referring to the Slug. Please scroll down to the bottom of the page and you should see a section with the URL and the Slug below it.
This error appears when you are trying to save a new record with a Slug that is identical to an existing Slug. All Slugs have to be unique.
We would recommend checking to see if there is another record with an identical Slug and edit it.