Did you add this when declaring your parent component? :

<script>  
export default {  
  props: {  
    block: String,  
  }  
};  
</script>  

Then to use your parent component you add:

<template>
  <ParentComponentNameHere :block="'Example'">
    <template v-slot:default="{ block }">
      <p>Block: {{ block }}</p>
    </template>
  </ParentComponentNameHere>
</template>

Also notice, it is {{block}} not {block}