Have you created textures to paint on? You have to have bitmaps loaded in the channels you want to use in order to start painting.
The quickest way to get going with BP is to use the Tools>Paint Setup Wizard. It will allow you to create UVs if they aren’t already set up, and it will also create bitmaps in whatever size you want for whichever channels you want to use. Highly recommend you use that tool.