I created two VM, Server1 and Server2 then Installed Active directory and Failover clustering feature for them
Both servers are added to Same domain RDSQLServer1.com
Now, When Login to Server1, open Failover cluster manager > Validate Clustering > add node
Server1.RDSQLServer1.com is able to add into cluster
But, Server2.RDSQLServer1.com of server2 is not able to find it so it is not able to add
Please help