For detail analysis please see this link：
https://blogs.sap.com/2016/12/30/why-my-extension-field-does-not-appear-in-available-fields-list-for-extension/ For detail analysis please see this link：
https://blogs.sap.com/2016/12/30/why-my-extension-field-does-not-appear-in-available-fields-list-for-extension/