My Global Entry membership is eligible for renewal with expiration in just under a year from now. I decided that I wanted to go and get Nexus. So I logged into the new CBP trusted traveler program (TTP) website, which replaced the GOES website. To my surprise, the "add a trusted traveler program" button was grayed out on my account.

Update:
I discovered how you get past this issue and on to a Nexus application. Hit the "renew membership" button, which you may presume applies only to your existing Global Entry account. It takes you to a page where you may choose between renewing the existing Global Entry membership or applying for the Nexus or Sentri programs.