<div v-for="(item, index) in items" :key="index">
...
</div>
<template> cannot be keyed. Place the key on real elements instead.
<form action="" class="form-star">
<template
v-for="(num, index) in 5"
v-bind:key="index">
<input type="radio" :id="'star' + (5 - num)" name="rating" :value="5 - num"/>
<label class = "full" :for="'star' + (5 - num)"></label>
</template>
</form>