Property “” was accessed during render but is not defined on instance.

When you see the error message “property ‘x’ was accessed during render but is not defined on instance”, it means that you are trying to access a property (in this case, ‘x’) in your Vue component’s template or methods that has not been defined within the component’s instance data or computed properties.

Here’s an example to clarify this error:

<div id="app">
  <p>{{ message }}</p>
  <button @click="updateMessage">Update Message</button>
</div>

<script>
new Vue({
  el: '#app',
  data: {
    message: 'Hello, Vue!'
  },
  methods: {
    updateMessage() {
      this.message = 'Updated message'; // accessing 'message' property
    }
  }
});
</script>

In the above example, we have a Vue component that displays a message in a paragraph. When the button is clicked, it triggers the `updateMessage` method that tries to update the `message` property. However, if the `message` property is not defined in the component’s instance data or computed properties, you will encounter the mentioned error.

To resolve this error, you need to make sure that any property being accessed directly in the template or in the component’s methods should be defined in the `data` or `computed` properties.

Here’s an updated example where the `message` property is properly defined:

<div id="app">
  <p>{{ message }}</p>
  <button @click="updateMessage">Update Message</button>
</div>

<script>
new Vue({
  el: '#app',
  data: {
    message: 'Hello, Vue!'
  },
  methods: {
    updateMessage() {
      this.message = 'Updated message'; // accessing 'message' property
    }
  }
});
</script>

In this updated example, the `message` property is defined within the `data` object of the Vue instance. Now, the `updateMessage` method can successfully update the `message` property without encountering the error.