An Error occured while starting Mirroring.

Additional information:

Alter failed for Database 'testbest_DB'.(Microsoft.Sqlserver.Smo)

An Exception occured while executing a Transact-SQL staement or Batch.(Microsoft.SqlServer.ConnectionInfo)

The server network address "TCP:dbserver1:5022" can not be reached or does not exist.Check the network addess name and that the ports for local and remove endpoints are operatioanal

I am getting this error while star mirroring,