You can check the Amex T&Cs for full details but the short of it is:
”Tickets are only available in L, U, T, X, and V classes of services for Main Cabin travel. For Delta Comfort+ travel, tickets are available in W and S classes of service, but only when L, U, T, X, or V classes of service are available in the Main Cabin. For Delta Premium Select travel, tickets are available in A and G classes of service. For First Class travel, tickets are available in I and Z classes of service. ”