System.FormatException: 'Could not parse the JSON file.' in C# .NET core 3.1

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I am doing web application using C# .NET core 3.1, the code can run and the web page is working when I built this application in the beginning; and I download 3 packages Microsoft.EntityFrameworkCore.SqlServer Microsoft.EntityFrameworkCore.Tools Microsoft.VisualStudio.Web.CodeGeneration.Design; and connect the sql server to my application, then the code still can run but the error appears.