There are two tabs: scene and game. You are in game. Put the [scene] tab and you will be able to draw.
(“Brushes are read-only” means non-modifiable brush. It has no effect in terms of drawing)
I am having the same issue. It was working fine, unitl I added a terrain kit. And suddenly my brushes went to read only. Thus far, noone has had a solution on any support site I have been on. It is very frustrating. If you found a fix, please let me know. Thanks so much!