Federating of ArcGIS Server with Portal for ArcGIS will fail if TLS v1.0 is disabled on the respective servers.
Last Published: March 23, 2016ArcGIS for Server
Bug ID Number
January 21, 2016
July 28, 2020
ArcGIS for Server
Operating System Version
This issue was not reproducible when tested by the development team. Issues may be given this status when they cannot be reproduced or are no longer relevant in a development version of the software, but a specific fix was not installed to address the issue. The issue's Additional Information section may contain further explanation.
User error. Manually editing the ArcGIS for Server configuration files is unsupported. The steps below would not address the root of the question. The issue is in Portal as a client, not as server.