Property or method "nation_list" 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.

This error message typically occurs when you are trying to access a property or method that is not defined in the component's instance. To resolve this issue, you need to ensure that the property or method you are trying to access is declared in the appropriate place.

If you are using the Vue.js framework, here are a few possible solutions:

1. Declare the property in the `data` option: Make sure that the `nation_list` property is defined in the `data` object of your component. For example:

data() {
  return {
    nation_list: [], // or initialize with the appropriate value
  };
},

2. Compute the property: If the `nation_list` property is derived from other reactive properties, you can use a computed property to define it. For example:

computed: {
  nation_list() {
    // compute and return the value of nation_list based on other reactive properties
    // ...
  },
},

3. Define a method: If `nation_list` is intended to be a method, make sure it is declared within the `methods` object. For example:

methods: {
  nation_list() {
    // implement the logic for nation_list here
    // ...
  },
},

By following one of these solutions, you should be able to resolve the error and access the `nation_list` property without any issues.