By the sounds of the reply he's not saying the 520 does not "do post codes", he's saying the post code you are trying to input isn't associated with a road name, or at least this is what he thinks you're trying to do.

For example, EN9 3BL is a post code on a road called Honey Lane. But sometimes you get a post code that doesn't have a road name - it can be on a utility road or private road or industrial park, or even a motorway or major A road.