I am having a problem where I create a polygon shape and then apply a brush to it. But what I am getting is one corner is not closed or linked together. I have no idea how to fix this issue. Is there a way to solve this problem?
The brush you are using is an Art Brush. Art Brushes just take an object and wrap it along the path from start to finish, even if the ends don’t line up perfectly. What you should use here is a Pattern Brush, which you can configure to fit perfectly. Here is a comparison between an Art and a Pattern Brush I made from the two rectangles on top.
If you want the Art Brush to look as if it closes, you just have to move the start/end points of the path to a straight line. You can easily achieve this by breaking the path open with the Scissor Tool.