Establish trust between ArcGIS Server and Portal for ArcGIS by importing ArcGIS Server's internal SSL certificate into Portal for ArcGIS. To determine the internal certificate used in ArcGIS Server, follow the steps provided below.
Log in to the ArcGIS Server Administrator directory. The following is an example of the ArcGIS Server Administrator Directory URL:
Click machines. Under Machines, click the appropriate machine hosting the certificates.
Identify the Web Server SSL certificate, and click sslcertificates.
Click the certificate, which is set as the Web server SSL Certificate. Click export and save the file.