The error message is pretty explicit:
Value type string can't be converted to system.data.SqlClient.SqlConnection
And it means exactly what it says.
Look at your code:
Dim SqlConnection As String = ...
Dim Check_User_Name_Found As New SqlClient.SqlCommand("SELECT ...", SqlConnection)
SqlConnextion is a String, not an SqlConnection instance.
Somewhere, you need to create an SqlConnecion instance using the connection string you read from your ConfigurationManager and open it. You can then use it to create your SqlCommand instance - but you can't just hand it a string and hope that it works, because it won't.
And a quick check says that you were told this 13 hours ago:
Sql connection error[
^]
The problem hasn't changed, the solution hasn't changed - you need to think about what you are doing and change your code!
Using con As New SqlConnection(strConnect)
con.Open()
Using cmd As New SqlCommand("SELECT iD, description FROM myTable", con)
Using reader As SqlDataReader = cmd.ExecuteReader()
While reader.Read()
Dim id__1 As Integer = CInt(reader("iD"))
Dim desc As String = DirectCast(reader("description"), String)
Console.WriteLine("ID: {0}" & vbLf & " {1}", iD, desc)
End While
End Using
End Using
End Using