In the PC app, when you import third party maps or charts, such has BSB format, it first creates a header-only qct file which does not contain any image data. When you open the map, it converts the image data and creates the full qct.
The Mac app does not have the ability to import third party maps, but it can open qct format maps that have been imported using the PC app.
The message "This map is a catalog header and has not been completely converted" is displayed if you open one of these header-only qct files in the Mac, iOS or Android app.
If you are using a folder that is shared between the PC and Mac apps, the way to fix it is to use "Refresh Map List" on the PC and enable "Load All Maps Now".