Brush not working

The brush tool doesn't work if there are no brushes in the file ('Basic' here is the equivalent of 'none'). 

Files created in Illustrator originate from a document profile that (usually) has a set of brushes already loaded in it.

If you open a JPEG or PNG directly, there won't be any brushes loaded, and thus you won't be able to draw with a brush until you add some.

Illustrator isn't meant to draw on pixel images. This is Photoshop stuff. You may want to educate yourself about some basics by reading the online help. If at all you would draw on top of your image by creating a new layer.

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