warn]: Property or method "fullName" 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/guid

这个警告一般是因为在模板中引用了一个不存在或未被定义为响应式属性的变量。你可以通过在组件的 data 选项中定义 fullName 属性来解决这个问题。例如：

<template>
  <div>
    {{ fullName }}
  </div>
</template>

<script>
export default {
  data() {
    return {
      firstName: 'John',
      lastName: 'Doe'
    }
  },
  computed: {
    fullName() {
      return `${this.firstName} ${this.lastName}`
    }
  }
}
</script>

这样就可以在模板中使用 fullName 属性了，并且它是响应式的。