Ok, I'm pretty sure I know what has happened. It all boils down to that path being part of a group. Technically, each path is in a Group of 1, which usually happens when there has been a file conversion, somewhere along the way (which you may or may not be aware of).
When I was working with your test file, where the groups had been eliminated (when you made the test file) duplicating the path solved the problem. But you weren't working in the test file, you were still in the original file, where I had no idea there was a group. You were probably duplicating the whole group, instead of just one path. Or you might have duplicated the path, but later selected it as a group.
In any case, here's what you need to do, starting with the original file, or a backup, or some file which doesn't have a lot of trial and error.
1 -- Ctrl + A (select all)
2 -- Object menu > Ungroup (or there's a button on the command bar
, or there's a key shortcut)
Now you will be able to follow the instructions from the manual, and everything should work fine.
You might have some trouble with aligning the text to the place you want on the path. For me, the text is going onto the path backwards. I think you already learned that you can reverse the path to fix that? Also, if you have trouble aligning the text to a particular part of the path, try un-centering it. Only use center, if you actually want it centered
Let us know how it goes
