Generally, it is
surface area configuration error or
sql service configuration issue. Have a look at this thread for potential reasons and resolutions:
Resolving A network-related or instance-specific error occurred while establishing a connection to SQL Server[
^]
If needed, See steps for setting up Surface area configuration
here..
Here, look at the following blog entries related to the same:
Under the default settings SQL Server does not allow remote connections[
^]
It says:
I. Make sure that Remote Connections are enabled in Surface Area Configuration tool
II. Check Whether TCP/IP protocol is enabled in the SQL Server Configuration Manager
III. Make sure that SQL server Browser service is running