Unable to start the migration due to the following error:
Specified SSH key file does not exist. Please specify valid filename for private SSH key and restart migration
The SSH key is missing on the source server.
Follow instructions from this article in order to install the SSH key and perform a migration by using it:
Migration | Authentication by SSH (Linux)
Switch the Authorization directive to the User / password value, fill the Root login and Root Password fields and click the Prepare migration button.
Migrating via the Plesk Interface