When Amex transitioned from the EWC to EWC+ connection method the ability to use Quicken Bill Manager's QuickPay was lost. This is not an issue unique to Amex. It affects all banks that have transitioned to EWC+. Quicken has said they are working to identify and implement a means to be able to offer QuickPay, again, but there is no timeline for when that might happen.
For now, the only method for paying bills with Bill Manager is CheckPay (print and mail checks).
Other options include: