The WinRM client sent a request to an HTTP server and got a response saying the requested HTTP URL was not available (4378970)

When trying to connect to Exchange Mail Server using its URL or FQDN :

Connecting to remote server exchange.company.com failed with the following error message :
The WinRM client sent a request to an HTTP server and got a response saying the requested HTTP URL was not available.
This is usually returned by a HTTP server that does not support the WS-Management protocol.
For more information, see the about_Remote_Troubleshooting Help topic.

On Demand Migration for Hybrid Exchange (ODMHE) uses Exchange Web Services (EWS) via port 80 and 443 to access your mail server from the Internet.

To migrate data to or from Exchange 2010/2013/2016/2019, kindly make sure that Outlook Web Access (OWA) is accessible from the Internet, so the OWA server name can be used for accessing your Exchange Mail Server via EWS.

If you are not using HTTPS for OWA, then please enter the full URL for your EWS service which follows this format :

http://servername/EWS/Exchange.asmx

To access your Exchange Mail Server using its hostname, please get the correct URL to be returned in the "ExternalUrl" value via this PowerShell at Exchange Management Shell :

Get-WebServicesVirtualDirectory | Select Name, *Url* | FL

To access your mailboxes slated for migration, the Migration Service Account will also need to possess the Exchange Management Role of "ApplicationImpersonation".