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Well, Photoshop has answered your question already really...you can't paint with the 'history' brush if you have no history.

You should be able to paint with the normal brush but until you do something and populate the 'history' panel with some activity, the 'history' brush has no history to paint with....hence the 'null' sign.

Paint something with the normal brush tool first and then see if the history brush tool works after that.

Regards.

MrTom.