I'm not sure what issue you are having, because honestly I've never had an issue with buying stuff online, using points and promo codes.
Points are limited to item sold by Sears or Kmart, which means that if you are shopping online you need to make sure that it's not sold by a third party seller, which Sears and Kmart has many different people selling online.
If you would like to limit your search results to items only sold by Sears and Kmart you can do this. After searching for something, lets say a kids t-shirt, after you hit search, under the bar there is a tab called Sears Only (Kmart Only) and this will show you all the items that are sold by Sears/Kmart. If nothing shows up, then Sears/Kmart doesn't have anything like that. That also if you are shopping on a desktop or laptop, if you are shopping from using your mobile phone, tablet or using the app it works a little different.
After you search for an item, there is a little tab called Refine at the top. You will click on this, and drop down menu will show up, towards the bottom there is a tab called Sellers, you will then click on this and a list of Sellers will appear. The top two are either Sears or Kmart, you will then click on this and only the items sold by them will show. If there names do not show, then item you are searching for is not sold by them.
Promo codes also will only work for things sold by Sears and Kmart, and you have to meet the required amount to use them. That means if you are buying shoes for $39.99 and you have a discount code for $5 off $40 then you didn't meet $40 yet and that's why you hit a wall.
If you have 15% off tools and you are trying to use it on bed sheets, then that also will not work.
You also have to make sure the coupon you are trying to use can be used online, they will say online and in store when reading the details of the coupon. If it says online only, then you can only use it online and if it says instore then you need to go to the store and use it. You can't try an instore coupon online, it won't work because the rules of that code say it won't.
If you can't handle shopping online, maybe try calling Sears.com and tell them you have this code, and this much in points and need help trying to use them. You can also make a trip to the store, and ask for help and I am sure someone there will be happy to help you place an order online and I'm sure they will even show you how to go about shopping and working Sears.com or Kmart.com.