A low-cost health insurance co-op that covers about one in three Arizonans that have an Affordable Care Act marketplace plan won’t be allowed to sell health plans Sunday — the opening day consumers can purchase insurance — after the state of Arizona and the federal government took action against the entity. The Arizona Department of Insurance said Friday afternoon that Meritus Health Partners/Meritus Mutual Health Partners has been placed into “supervision” and only can continue to serve existing clients until the end of the year. The federal government also suspended Meritus from the Affordable Care Act’s federal marketplace, which means the company won’t be able to sell health plans via healthcare.gov when the health-care law’s three-month enrollment period begins Sunday.