When you successfully add a source server to the AWS MGN console, you will need to follow a lifecycle which is demonstrated here - MGN Lifecycle.

For example you can only launch cutover instances when the source server has been tested. If you try to mark the source server as "Ready for cutover" you will experience issues as MGN requires that the source server be tested before launching cutover instances.

Therefore, you will need to launch a test instance and then mark the instance as "Ready for cutover".

Hope this helps :)