First of all, it's not clear what you're trying to accomplish with this category. Categories are a way to group multiple profiles together. I'm not sure what other profiles you think would belong in the brick wall category for your Duncan Johnson.
If you want a personal brick wall category for all your brick wall relatives, the documentation for how to set that up is here:
https://www.wikitree.com/wiki/Help:Personal_Categories
In this case you might use a category like [[Category:Mallow-254 Brick Walls]]
You might also see if any of these other brick wall subcategories fit:
https://www.wikitree.com/wiki/Category:Family_Brick_Walls keeping in mind that, unless the subcategory is part of an active study (like a One Name Study) it will probably only ever attract one profile and will never be touched again.
Or you might decide that an app
https://apps.wikitree.com/apps/nelson3486/brickwalls/ is more useful. But again, without knowing how you intend to use the category, it's hard to suggest anything.