Unable to establish a secure connection to the layer. The layer cannot be added to the map

Error Message

When attempting to add an ArcGIS Server web service through HTTP to an ArcGIS Online web map through HTTPS, the following error message may be returned:

Error: 
Unable to establish a secure connection to the layer. The layer, [layername], cannot be added to the map.

Cause

The ArcGIS Online organization allows access through HTTPS only; therefore, content accessed through HTTP cannot be added to the web map.

Please see the following ArcGIS Online help topic: Configure security settings

Solution or Workaround

Add the GIS web service through HTTPS.

• If the GIS web service is not accessible through HTTP, enable SSL on the ArcGIS Server site. To enable SSL on the ArcGIS Server site, please see the following web help topic: Enabling SSL on ArcGIS Server
• If the GIS web service is going to be accessed through the web adaptor, also enable SSL on the web server hosting the web adaptor. To enable SSL through the web adaptor, please see the following web help topic: Enabling SSL on ArcGIS Server when accessed through ArcGIS Web Adaptor