If you have Free Shipping checked as a method you're making available, go to the free shipping tab and be sure to select the "Free Shipping Requires..." drop down and make sure not to choose an option that requires a coupon to be used at checkout unless you're using coupons.
This happened to me and that was the solution. I wasn't using coupons, but it was requiring a coupon code to be used, so I got the error. Hope that helps.