The path 'node/x' is either invalid or you do not have access to it. x=a number

I have installed Drupal 6.1 and DHTML Menu version 6.x.

I made a new menu and I have tried to add new items but I received the following error: "The path 'node/22' is either invalid or you do not have access to it." (please see the attached file).
I must tell you that I was able to create the new menu and the first 2 items in it. For the menu, I wrote to the Path: node/1, for first item: node/2 and for second item: node/3. After that, I tried to add item 3 (with path: node/4) and I got that error.
If I put any other number instead of 4 I get the same error message.

Please let me know where my mistake. Please be explicit because these are my first steps in Drupal.
Thank you for your support!