By clicking the Test link associated with a channel destination, you are effectively testing the user credentials provided by a wholesaler/distributor. Sporadically, your partners may update their FTP/SFTP et al services. It is incumbent upon you to update the destination accordingly when such changes are made.
Failed distributions with a message of upload failed are an indication that the destination is not properly configured. If the channel destination remains inaccessible for multiple days, contact the partner.
After clicking the Test link, you should either see an accessible or inaccessible response.