Strange error message running an Angular application

[Alejandro Barrero]

Just running an application from the Udemy course RxJS 6 in practice (I downloaded the file for rxjs-course-0perators). I anr running with "ng serve --open" the console in Google Chrome displays

Uncaught Error: Can't resolve all parameters for ApplicationModule: (?).
    at syntaxError (compiler.js:1021)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver._getDependenciesMetadata (compiler.js:10922)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver._getTypeMetadata (compiler.js:10815)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver.getNgModuleMetadata (compiler.js:10683)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver.getNgModuleSummary (compiler.js:10493)
    at compiler.js:10607
    at Array.forEach (<anonymous>)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver.getNgModuleMetadata (compiler.js:10595)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver.getNgModuleSummary (compiler.js:10493)
    at compiler.js:10580

I have not been able to find a file with so many lines.