An ArcGIS Server secure service that can be accessed through REST endpoint fails to be accessed or added in ArcGIS Online Map Viewer.
Although ArcGIS Server and ArcGIS Online may be configured to use HTTPS only, the website hosting ArcGIS Web Adaptor needs to be bound with a server certificate to use port 443. Binding refers to the process of configuring the certificate to use port 443 on the website that allows HTTPS connections between ArcGIS Server and ArcGIS Online through the ArcGIS Web Adaptor.
Log in to the Internet Information System (IIS) of the website hosting ArcGIS Web Adaptor, and bind the server certificate to port 443. For more information on creating a server certificate and binding it to a port, refer to ArcGIS Enterprise: Enable HTTPS on your web server.
Note: Users may have an existing server certificate, but it is not bound to ArcGIS Web Adaptor. For more information on using an existing certificate, see ArcGIS Server: Configuring HTTPS using an existing certificate.