Can't sign up without a device?

I'm trying to set up a plus plan, but the website directs me to the app. So I open the app to set it up, but it says I can't initiate a plan without a compatible device. I'm signing up to take advantage of the 10% discount offer. I can't get the discount without being signed up, but I can't sign up with a device, defeating the purpose of the discount in the first place. What do I need to do to sign up for the plus plan and get the discount on the products I'm trying to get for my home?