Check that you are on a supported template. The RuffRuff Table of Contents block only appears on the following templates:
Blog post template
Product template (Regular Plan)
Page template (Regular Plan)
Collection template (Regular Plan)
Blocks for Regular Plan templates will not appear if you are not subscribed to the Regular Plan.
Check the bottom-left area of the theme editor or go to Theme settings → App embeds.
If your theme supports OS 2.0, use the app block (add it inside a section) rather than the app embed.
Try uninstalling and reinstalling the app — this resolves the issue in some cases.