The error you get is most probably due to the fact that the system cannot establish connection the SQL server, that is configured in the Data.Config or the webconfig.
Please verify that your database is accessible and the connection string is correctly configured to match your installation, because the error you get is thrown because your SQL server is not accessible - remote connections are not allowed, TCP protocol is not enabled or the SQL service is down. You may find this link useful:
An error has occurred while establishing a connection to the server.
You can also check the following article:
Resolving A network-related or instance-specific error occurred while establishing a connection to SQL Server
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