Unable to add replica set to shard

I was trying to add a new replica set to my mongodb sharded cluster but I keep getting the following error: { "ok" : 0, "errmsg" : "Could not verify that config servers were active and reachable before write", "code" : 25 } I checked all the config servers and they are up and running.The problem is new as I can previously add shards without any issues. The MongoDB version I am using is 3.2 running on Ubuntu 16.02. How can I fix this?