Windows Media Player cannot play some or all of the items in the playlist (Windows Media file with an .asx extension) because it is nested. A nested playlist contains another playlist, which contains another playlist, and so on. The last playlist (the most nested one) does not contain any items.
If you created the playlist, you can either delete the most nested playlist or add items to it, and then try again.
If you encountered this error message after clicking a link on a Web page, the link might not be valid.