If you want to use an existing ECS as the target server, make the following preparations:
If the target ECS runs Linux, enable TCP port 22 for a file-level migration or ports 8900 and 22 for a block-level migration.
For details about how to configure security group rules, see How Do I Configure Security Group Rules for Target Servers?
If you indeed use the ECS as the target server, do not perform any operations on the ECS during the migration, such as stopping or restarting the ECS, attaching or detaching a disk, or changing the password, or the migration will fail.