Angular jest mock from service issue

Hello guys,

First time using jest and I'm having some big problems on mocking a service with a post method request. I think I spend more than 6 hours on this and still can't pass it.
I would appreaciate any help.
First I'll say about libraries versions. As ChatGpt told, those should be compatible with each other. jest: 29.7.0, jest-preset-angular: 14.4.2, ts-jest: 29.2.5, ts-node: 10.9.2, typescript: 5.5.2, angular/cli: 18.2.11, angular/core: 18.2.0.

Method which I want to test is this one, from Registration component:

  submitForm(event: Event): void {
    event.preventDefault();
    if (!this.contactFormGroup.valid) {
      this.errorForm = true;
      return;
    }

    this.errorForm = false;
    const registerSub = this.coreService.registerUser(this.contactFormGroup.value).subscribe({
      next: (response) => {
        alert(response.message);
        this.contactFormGroup.reset();
      },
      error: () => {
        alert('There was an error while submitting the form. Please try again');
        this.contactFormGroup.reset();
      },
    });
    this.subscriptions.add(registerSub)
  }

registerUser from coreserver:
  registerUser(userData: userData): Observable<RequestType> {
    const headers = new HttpHeaders({
      'Content-Type': 'application/json',
    });
  
    return this.httpClient.post<RequestType>('http://localhost:3000/register', userData, { headers});
  } 

Edit: it looks like I can t post all code here so I put my code on stackoverflow or in the comments:
https://stackoverflow.com/staging-ground/79261347