Ensure that the SQL Browser Windows service is running on the server and client protocols are enabled (TCP/IP preferred). Also check the port, the SQL Server is listening to (by default it's 1433). If using other port, add it to the instance name like
ITADMIN\SQL2014:1500
.
If you're using a firewall, be sure that the port is allowed.
Last idea: check if you are using the correct instance name. If not sure, open the SQL Management Studio, right click on the server node, choose Properties from menu and copy the instance name from dialog.