I have Failed to construct ‘FormData’: parameter 1 is not of type ‘HTMLFormElement’ on my devtools

I want to send my form with form data, method POST, with Fetch API in JavaScript but I have an error.
Why I have this : runtime-core.esm-bundler.js?d2dd:218 Uncaught TypeError: Failed to construct ‘FormData’: parameter 1 is not of type ‘HTMLFormElement’.

My project is based on vue.js, express.js and mysql.
I haven’t started the backend part of the function signup in (router/controllers). But normally nothing to do with the error that is displayed

When the request returns me a code 200 at this time I can make my backend which uses express.js.
Thank you for your help.

//Vue.JS

<script>
export default {
  name: "FormSuscribe",
  data() {
    return {
      email: " ",
      password_key: "",
    };
  },
  methods: {
    submitForm() {

      const formData = new FormData(this);

      fetch("http://localhost:8080/api/auth/signup", {
        method: "POST",
        body: formData,
      })
        .then(function (response) {
          return response.json();
        })
        .then(function (json) {
          console.log(json);
        })
        .catch(function (error) {
          console.log(error);
        });
    },
  },
};

</script>

<form
    method="post"
    @submit="submitForm()"
    class="d-flex flex-column justify-content-center align-items-center"
  >
   

    <div class="form-group col-lg-5">
      <label class="form-control-label">Email</label>
      <input
        type="text"
        class="form-control"
        id="formGroup2"
        placeholder="@"
        v-model="email"
        name="email"
        required
      />
    </div>
    <div class="form-group col-lg-5">
      <label class="form-control-label">password</label>
      <input
        type="password"
        class="form-control"
        id="formGroup1"
        placeholder="*******"
        name="password_key"
        v-model="password_key"
        required
      />
    </div>
    <div class="form-group">
      <button type="submit" value="Submit" class="btn fw-bold mt-4">
        Inscription
      </button>
    </div>
  </form>