If you can’t use the enrollment button, it may be because the plan is “Part D Suppressed”. With Part D Suppression, there has been an inaccuracy in a plan’s drug pricing submission. The enrollment button will be disabled ONLY if you do a search with drugs. If you do a search without drugs, the enrollment button can be used. Plan Finder discourages enrollment into plans that have submitted inaccurate drug prices.

SOURCE: https://www.medicare.gov/find-a-plan/staticpages/faq/planfinder-faq.aspx