[Vuejs]-Child not updating based on props in Vue.js

Are you getting any error. As I can see Map.has has poor support in browsers. You can try using any of following alternatives:

post in errors

<template>
    <div v-if="'post' in errors" class="alert alert-danger" v-text="errors.get('post')"></div>
</template>

errors[‘post’]

<template>
    <div v-if="errors['post'] !== undefined" class="alert alert-danger" v-text="errors.get('post')"></div>
</template>