custom-input-6.js

<!-- CustomInput.vue -->
<template>
...
    <p v-if="error">{{ error }}</p>
...
</template>

<script>
...
  data() {
    return {
      error: '',
    };
  },
...
    onInput(event) {
      const value = event.target.value;

      if (!value) {
        this.error = 'Value should not be empty';
      }

      this.$emit('input', event.target.value)
    }
...
</script>