How to use Content Complete Module?
the Master display is the 1st Views page that you edit when making a new View. Typically this is a Page or Block in Drupal7. So you have a View somewhere that if you try to save it the View will say you need to supply at least 1 field for the completeness data field. So I suggest going through all of your Views and trying press Save on each 1 (because you dont know which 1 it is apparently or what exactly we're talking about) -- 1 will complain it cannot be saved until you fix this error in it.