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.
Dude this is rocking...minor issue with body column witdhs on the index page, but when I replaced the adm_pic.jpg with one of my own it got all magically corrected and all. Kind of awesome. Easiest install, cudos to you. One other minor issue...on the Index page when the CONFIGURATION menus drop down (Google Chrome) the TOTAL REVENUE graphs obscure part of the drop down menu. Not a real big issue...just thought I should let you know.