My gas furnace won't turn on all it does is flash rapidly it says it's a reversed 115 vac polarity/verify gnd

First verify that the system is grounded by testing hot to ground through a meter. It should read 115-120VAC. Is the black hot or white? Then, you have an apparent reversal of the black and white wires. Is this something new? Just happened?

If you've not done or had done any electrical work, chances are you have a bad board.