If you went directly from 2.6 to 2.9 then that is a problem.
Each update needs to be installed, in order. The error you are getting is due to not applying the 2.7 update where the table is altered to contain the new column.
I hope this helps...also, I believe this thread is for the other 'Discount Coupon Codes' and not 'Discount Code' module the one done by the high quality php guys...of which I think you are installing.