Unable to use the United Kingdom (UK) postcode data to create map boundaries.
Use one of the following workarounds.
Use UK Postcode Districts (outcodes), the first two-to-four-character segment of the five-to-seven-character UK postcode, to create map boundaries.
Use Custom Geocoding
to plot the desired data on a map.
Tableau Desktop contains several built-in geographic roles with predefined longitude and latitude coordinates. Currently, the built-in geographic roles only include the UK outcodes.
Some custom geocoding resources are available from the Information Lab: