I would like to express my sincere appreciation for discovering this solution. Should you encounter any additional concerns or require further assistance, please do not hesitate to reach out to us. We are here to provide prompt and professional support.
We have investigated the issue you reported and found that the page where you want to add a table of contents is not a specific single blog page or post page. It seems that the page is a dynamic page where you are displaying some specific posts. Unfortunately, it is not possible to show a table of contents on such a page.
If we have misunderstood your request, please let us know.
We appreciate your patience and cooperation.