Since Comcast doesn't care about you, I just wanted to let you know that this is due to a bug in their system. If you have the Flex device, you are prevented from changing anything in your account. The solution is apparently to return the Flex streaming device, so contact Comcast and tell them that you'd like to return it so that you can browse your options and see what's available.
I also see that you posted this problem in August. It's now February, and the problem remains.
Hint: They aren't going to fix it.