I'm trying to work my way through the "Ultimate Community Site Guide", and am trying to create a Panel Page (p32), but cannot get the "node/%" path to work.

Page 32 says:

Here is the tricky part: set the path to “node/%”, by doing this all of
the node/3, node/4, node/5, . . . pages will be replaced by this panel,
and the argument % will be passed to the panel.

Unfortunately, I get the error message:

You cannot have an unnamed placeholder (% or ! by itself). Please name your placeholder by adding a short piece of descriptive text to the % or !, such as %user or %node.

I've also tried "node/%node" as the path but I then get the following error message:

That path is already in used. This system cannot override existing paths.