H1 is switched off by default, and that is usually right: on most themes the only H1 is the post title, so a contents list built from H1s would have one entry — the page you are already on.
If your content genuinely uses H1 for its sections, tick H1 under Settings → Heading levels. It is worth fixing the content instead if you can, because several H1s on one page is a real accessibility problem independent of this plugin. In the block editor the post's Table of Contents box offers to do it: with two or more H1s it shows Make them H2, which moves the headings under them down with them. Undo, or Ctrl+Z, puts them back, and nothing is saved until you save the post.
Still stuck? Ask the developers who wrote it. We aim to reply within one business day.
Table of Contents
Give a long post a contents list built from its own headings, and a permanent link to every section.
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