I am attempting to make a simple pattern brush with five b&w rectangles, with no success. Each time I drag the artwork to the brush swatch and i get the message seen below. I have tried various things like deleting some of the objects, but it fails except where there is one square white rectangle.
I don't know what you did wrong the last time but I can assure you, white artwork or multi-color artwork has nothing to do with it...You can have whatever/as many colors as you like.
You should give it one more try.
Look here :
Part of the old answer:
You should also try using Options -> Expand [or Expand Appearance]. This will help if you have any unsupported attributes.