Connection database network-related or instance-specific errors occur when connecting to SQL Server, verify that the instance name is correct and that SQL Server is configured to allow remote connections.

If your connection string is the same

<add key= "ConnectionString" value= "server=127.0.0.1;database=newstest;uid=sa;pwd=***"/>

or something like that.

<add key= "ConnectionString" value= "server=local;database=newstest;uid=sa;pwd=***"/>

And there is no grammatical error, it is recommended to try the way below

<add key= "ConnectionString" value= "server=.\sqlexpress;database=newstest;uid=sa;pwd=***"/>

Which is to put server=127.0.0.1 or server=local as server=.\sqlexpress.

Hope to be able to help the people who meet this problem.