Hi Tai_Van,
Please follow the instruction on this website and make sure all required extensions are installed: https://code.visualstudio.com/docs/other/unity
If the issue persists, please check the console of VS Code (not Unity!). If the .NET Framework 4.7.1 (Developer Pack) is mentioned there, download and install it from the official Microsoft website. Here is the link: https://dotnet.microsoft.com/download/dotnet-framework/net471
Install and use Visual Studio (Community Edition). For some students, VS Code simply does not work well. Maybe due to a problem with Unity, some update or whatever.
- Download VS and follow this instruction.
- Select VS as your External Script Editor in Unity (Edit > Preferences > External Tools).
- Install the “Visual Studio Editor” package in the Package Manager.
That should work. 
See also:
- Forum User Guides : How to mark a topic as solved