You start by going to Site Action/Site Settings and then under the Look and Feel section, you clicked the "Navigation" link.
Editing the navigation as in this screen shot, you clicked "Add Link".
Now you try to add the relative URL as in this screen shot, but you get the error depicted. What gives?
Well, it's quite simple really. The answer is that you're editing a sub web of a Collaboration Portal. Collaboration Portal sub webs have different navigation functionality in the Quick Launch than normal Site Collection sub webs do.
If we repeat the process, but this time on a Site Collection sub web instead, it would look like this…
Editing the menu you'll see a much different interface:
And adding a relative URL is fully supported as can be seen here:
Of course, that leaves you begging the question, so how do I change one site for the other?
Simply follow these steps:
- Save the sub web as a template.
- Download the .stp file from the Site Template Collection library.
- Navigate to the target site collection root site.
- Through SA/SS access the Site Template Collection library.
- Upload the .stp file to the library.
- Create a new sub web using the newly uploaded template.