SPD Error: Microsoft SharePoint designer does not support editing non-SharePoint sites

Error:
"Microsoft SharePoint designer does not support editing non-SharePoint sites" - while trying to open a SharePoint 2016 site with SharePoint designer 2013, got this error message.

Solutions:

1. Simply close all instances of your SharePoint Designer and then reopen it.
2. Check if client integration is enabled https://support.microsoft.com/en-us/kb/981223
3. Clear SharePoint designer cache. Here is How to Clear SharePoint Designer Cache
4. Try re-installing SharePoint designer!