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 in Web Application settings from the Central Admin site.
3. Clear SharePoint designer cache. Here is How to Clear SharePoint Designer Cache?
4. Try re-installing SharePoint designer!