How to fix: "the selected illustraton contains an element that can´t be used to make a brush pattern

Select the bicycle chain then go to Object -- Expand

Don't use Gradient to the object. If you used it then Delete it and try again... It works for me.

Can you show something?

Maybe it contains another Pattern.

The artwork cannot contain gradients, blends, other brush strokes, mesh objects, bitmap images, graphs, placed files, or masks.

For art and pattern brushes, the artwork cannot contain type.