If you delete a message from a user on another instance, it is only deleted locally on your instance, or rather, only from your local timeline. That is why you also get the message

‘Conversation Not Found
Unfortunately, the requested conversation isn't available to you.’
The message remains visible to all other users on your instance and on other instances. Unless you delete it via /moderation/item/delete, in which case it will no longer be visible to users on your instance, but it will still be visible

Thanks @OldKid ⁂ I deleted the message like you said, and it's actually no longer on my timeline /moderation/item/delete . it worked👍

Just to clarify
The message they wrote on the time line, even if I deleted it, was always present on my time line (I am not a user but an Administrator)

Sorry if I reply to you in this thread , The conversation where you replied to me no longer exists for me and I cannot reply,I had deleted it and created a new one, it has disappeared from my time line but not on the Friendica forum, I don't understand , this is what I see , the first post I created doesn't exist , but with the direct link to the friendica forum I see my first post. Thanks for the help