Property or method "index" is not defined on the instance but referenced during render. Make sure that this property is reactive

This error usually occurs when you are referencing a property or method in your Vue component's template that is not defined in the component's instance. To fix this, make sure that the property or method you are using in the template is declared and defined in the component's `data`, `computed`, or `methods` sections.

If the property is supposed to be reactive, you can define it in the `data` section of your component. For example:

data() {
  return {
    index: 0
  };
}

If the property is a computed property, you can define it in the `computed` section. For example:

computed: {
  index() {
    // compute and return the index value
  }
}

If the property is a method, define it in the `methods` section. For example:

methods: {
  index() {
    // perform some calculations or actions and return the index value
  }
}

Make sure to check for typos or incorrect naming when referencing the property or method in your template.