Property "grades" was accessed during render but is not defined on instance.

This error message typically occurs in a Vue.js application when a component tries to access a property that has not been defined in its instance data.

To fix this error, you will need to ensure that the property "grades" is defined in the component's data object. For example:

<template>
  <div>
    <ul>
      <li v-for="grade in grades" :key="grade">{{ grade }}</li>
    </ul>
  </div>
</template>

<script>
export default {
  data() {
    return {
      grades: ['A', 'B', 'C']
    }
  }
}
</script>

In this code, the component's data object includes a property called "grades" that is an array of strings. The component's template then uses a v-for loop to iterate over the grades array and display each grade in a list item.

By defining the "grades" property in the data object, you should be able to access it in your component without encountering the "grades" not defined error.