how can i do it? i cannot find anything about it in this forum, but i saw that it is possible

Install and use moving replies add-on.

oh, thanks.. i didn't looked for it in the plugins list

I went to the topic I wanted to move a post to, I found the id = 19

I went to the post and clicked "move"

The post field auto filled. I input the 19 for the topic field.

I get this error:
Either post or topic do not exist, or you have specified forum or message ID incorrectly. Click BACK button in your browser ang correct your input!

The error means what it means. Make sure you have enter the existing topic's ID, and not messed it up with the forum's ID.

Here how to locate topic's ID.

As usual, it was me not understanding clearly. The forum had no topic yet, so I created one. I then used that topic's ID. Thank you.
gare

If you would like to move a post to a non-exiting topic, you may create this topic right away, specifying its title and the forum from the dropdown menu of the same add-on.