NullInjectorError: No provider for JwtHelperService

You need to use JwtModule provided by the @auth0/angular-jwt, which will add JwtHelperService to the providers, or you need to add it manually to the modules provider.

Something like

const JWT_Module_Options: JwtModuleOptions = {
    config: {
        tokenGetter: yourTokenGetter,
        whitelistedDomains: yourWhitelistedDomains
    }
};

@NgModule({
    imports: [
        JwtModule.forRoot(JWT_Module_Options)
    ],
...

Fore more see Documentation

For future reference, if all you want to use JwtHelper for is decoding, like in this case checking if the token is expired, then you can use this.

import { JwtHelperService } from '@auth0/angular-jwt';

const jwtHelper = new JwtHelperService();

@Injectable()
export class AuthService {
    public isAuthenticated(): boolean {
    const token = localStorage.getItem('token');

    // Check if the token is expired and return true or false
    return !this.jwtHelper.isTokenExpired(token);
}

Source: Documentation

A little late to the party, but I ran into the same issue trying to follow the standalone docs and what it doesn't cover is the need to import the options InjectionToken which is referenced in the constructor of the service:

import { JwtHelperService, JWT_OPTIONS  } from '@auth0/angular-jwt';

...

providers: [
        { provide: JWT_OPTIONS, useValue: JWT_OPTIONS },
        JwtHelperService
    ]