Question

Can please some tell me when does the above written error occurs. I just wanted to get more information on this.

Answer 1

Check the config file where you mentioned the connection string and correct it…
The database mentioned might not be a valid one, or the server name might not be valid.
Your connection string should look like this

Server=myServerAddress;Database=myDataBase;User ID=myUsername;Password=myPassword;Trusted_Connection=False;