For information on using audio description on iPlayer, have a look here: Are programmes with audio description available?
Sometimes only the audio described or signed versions of a programme are available on BBC iPlayer. The audio description is built into that version and can't be removed.
If you can't find the non-audio described episode, it's because that version has already been available but has now expired.
The accessible version of a programme is often broadcast a few days after the standard version, so there will be a period where the AD or sign language versions are the only versions available.
