Check the replication subnet to see if the replication server exists. If it does it's probably a communication issue between the replication server and MGN. Check routing, NACLs, and Security Group. https://docs.aws.amazon.com/mgn/latest/ug/Network-Requirements.html
If there's not a replication server, its creation might be blocked by Service Control Policies or Control Tower Guard Rails (For instance, I've seen tagging requirements block the creation of the replication server). Check Cloud Trail and Cloud Watch for hints.
Check your settings under the Application Migration Service in your region. Verify that the subnet in the settings is the expected one.
Hi! Check the ports that AWS MGN require in order to succeed with the replication. MGN uses TCP 443 & 1500. Do another check on the "security group" rules and ensure it is configured properly. Check the source server that has enabled the ports 443 & 1500. In case you've an external firewall, ensure to do the same thing. Perform some tests to check the ports are fully open. Not sure which OS you're using but if it's a Windows Server use command "netstat -ab" or if it's Linux (depending the distro) you can use "Debian = sudo ss -tlpn & netstat -tlnp" and/or "RedHat = sudo netstat -tlnp & sudo lsof -i". There is a lot of ways to test the connectivity on your servers and ports enabled through Linux, but also you can use telnet to check for the same. In case you've any other difficulties, I'll be glad to help you.
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