I am trying to create a post that anyone is able to see, including guests that have not registered.
I have changed the permission for people to see the post to the lowest rank (guest)
But I have tried viewing the forum post myself logged out of my account viewing as a guest, but still saying I am not authorised.
To make a forum post visible, a visitor to your site would need permission to view the Forums Section of your site; to check this go to Site Admin -> Permissions -> Guest Account Type and ensure the "View Forums" permission is checked.
They would also need permission to view the forum itself the post resides in; you can check this by going to your forums page and clicking Admin and checking that the Forum has the "Read View Level" set to guest.
Finally, if you enabled thread permissions for the forum, you would need to ensure that thread has the "Read View Level" set to guest.
I hope that helps =)