Hi,
Please try the below,
1.Check the Firewall communication between the existing and the new member - cluster port, etc - Check using telnet
2.Pre-requisites are properly installed on the second node trying to add
3.Uninstall the windows failover cluster from server manager, reboot the server. Install the FailoverClustering role and then try adding the node to the member
4.Turn off windows firewall if its on and try
If you still face the issue, share the DAG tasks logs for the inner exception.
If the above suggestion helps, please click on "Accept Answer" and upvote it.