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Kakashi.
If it is the first post of the topic, is normal: you can't delete the first post, you need to delete the whole topic (from the Moderación menu at the top of the page).
If you want to delete the first post, you need to select it (from the checkbox at the bottom right) and then go to the Moderación menu at the top of the page and choose Remover los posts seleccionados..