Hope you are doing well.
The first thing that I would check is if the post is picking the right template by adding a die('some message')into the template file.

The second thing that I would check is if the comments for the post are open. Remember that the comment_form() method will do nothing if comments are closed.

Hope this helps to spot the issue. Please keep me posted.

Cheers

On the first point, I don’t know exactly what you mean, but everything seems to be working well. Because the page displays very well. It is only the comment form that is not displayed.

On the second point, I verified and comments are open. And it works very well on front-page.php. So, I’m lost

For the second point, I understand that you are using this template for a page.
Remember that comments are disabled in pages by default, even when the comments are enabled for the site. Make sure that comments are allowed for the specific page.
Check this screenshot for further clarification https://ibb.co/cGFQee

Thank you for your help. I’ve been looking all day. But it was very simple. I feel stupid :). I never knew that comments are also activated in the dashboard when publishing.

Really, thank you. I will therefore mark the subject “resolved”.