Reactive properties error in vue-cli

hello,

I have a few problem with vue-cli. I try to display (in the main component) the text which are enter in the input (in the child component). it's work (so strange) but there are an error message :

vue.esm.js?efeb:578 [Vue warn]: Property or method "test" is not 
defined on the instance but referenced during render. Make sure that 
this property is reactive, either in the data option, or for class-
based components, by initializing the property. See: 
https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-
Properties.

found in

---> <Signup> at src/components/auth/Signup.vue
       <App> at src/App.vue
         <Root>

I search on internet and there a lot of example to resolve this error but not with the architecture vue-cli. I don't understand that...

Step by step :

I write a component :

<template>
   <div class="container">
     <p>{{ data.test }}</p>
     <form @submit.prevent="signup">
      <v-customInput v-model="test" @onChangeValue="onChange"></v-customInput>
     </form>
   </div>
</template>

<script>
  import CustomInput from '../shared/CustomInput'

 export default {
   name: 'HelloWorld',
   components: {
    'v-customInput': CustomInput,
   },
   data() {
     return {
       data: {
         test: '',
         first_name: '',
         last_name: '',
         email: '',
         password: '',
         vat: 0,
         creation_company_date: new Date(),
         phone: '',
       },
       error: false,
     };
    },
    methods: {
     onChange(variable) {
       const data = this.data;
        for (let value in data) {
         if (value === 'test') {
           data[value] = variable;
         }
        }
     }
   },
 };
</script>

and a child Components :

<template>
  <div class="customInput">
    <input v-model="value" type="text">
    <label>First Name</label>

<script>
 export default {
  name: 'CustomInput',
  data() {
    return {
      value: '',
    };
  },
  watch: {
    value: function(val, oldVal) {
      this.$emit('onChangeValue', this.value);
    }
  },
 };
</script>