Localhost refused to connect in the hello world demo

I am working on a school project which is developing a Microsoft Teams web app. I followed the instructions given in the https://docs.microsoft.com/en-us/microsoftteams/platform/build-your-first-app/build-and-run to create a hello world demo. But when I run the project in VS code I met these error messages:

2020-11-28T15:25:39.764Z AppCache: App Cache is not defined on the desktop app
statics.teams.cdn.office.net/hashedjs/0-angular-jquery.min-eee9041.js:114
2020-11-28T15:25:39.810Z transportDependencyProviderService: transport can't be initialized, settingsService is enabled - true,
          desktop is pluginless - false,
          destop is Hwa - true,
          is desktop client - false,
          slimcore enabled - false


Error: Failed to resolve: the task is already resolved
    at o._checkState (https://statics.teams.cdn.office.net/hashedjs/3.1-app.min-5d09005.js:1:5680141)
    at o.resolve (https://statics.teams.cdn.office.net/hashedjs/3.1-app.min-5d09005.js:1:5678949)
    at Object.successFunc (https://statics.teams.cdn.office.net/hashedjs/3.1-app.min-5d09005.js:1:5895803)
    at https://statics.teams.cdn.office.net/hashedjs/3.1-app.min-5d09005.js:1:5681654
    at a (https://statics.teams.cdn.office.net/hashedjs/3.1-app.min-5d09005.js:1:5675968)
    at o._resolveCallback (https://statics.teams.cdn.office.net/hashedjs/3.1-app.min-5d09005.js:1:5681633)
    at https://statics.teams.cdn.office.net/hashedjs/3.1-app.min-5d09005.js:1:5681525
    at Array.forEach (<anonymous>)
    at o._resolveSuccesses (https://statics.teams.cdn.office.net/hashedjs/3.1-app.min-5d09005.js:1:5681443)
    at o._addCallbackSet (https://statics.teams.cdn.office.net/hashedjs/3.1-app.min-5d09005.js:1:5677955) Unhandled error caught in promise chain

I also put the screenshots of the errors here as it might be easier to read with all the informations:

I have been able to add it into teams, but it displays "localhost refused to connect" under the "Personal Tab"; and no response returned from the chatbot.

The `localhost:3978` displays Cannot GET /. 

I am sure that the `nvm install` and `nvm start` has run successfully.

The whole project is generated by VS Code Microsoft Teams toolkit. Is is the problem of my network? Do I need to modify any settings for this project? Can some one tell me how should debug this problem? Thanks!!!