Eu encontrei este post que poderia ajudar.
Se você rolar para baixo um pouco, duas respostas realmente úteis serão exibidas (não quero agir como se fosse meu):
sarah postou:
i had that problem on illustrator cs4 (circle with a slash through it when i chose paint tool) and found that the brushes were not loaded. go to the right hand brushes icon on your toolbar and pop it out. then click the down arrow and scroll down to load calligraphic brushes. then you can browse for a brush style, all of a sudden you are able to find all your colors and paint on your image. hope this is helpful
GizmoJones postou:
Go to the menu and select Window –> Brushes to open the little “Brushes” window. If it’s empty, you haven’t chosen a calligraphic style for your brush. Click on the arrow in the corner of the “Brushes” box, and select “Open brush library.” Pick a style, which will show up as a tab in the box, and your brush will work.
Espero que isso ajude você a usar a ferramenta pincel novamente, vivas!