State Error CS1069 The type name 'SqlConnection' could not be found in the namespace 'System.Data.SqlClient'. This type has been forwarded to assembly 'System.Data.SqlClient,
Most likely the System.Data.SqlClient.DLL in not in the Bin folder, and you have not set reference to the DLL in the project. You have missed the database choice when You install visual studio. And now You just manually add references named "System.Data.SqlClient".
1.right click you project name and choose nuget package options.
2.search "System.Data.SqlClient" and install it.