[Vue warn]: Avoid using non-primitive value as key, use string/number value instead

Other answers work, but the following is also worth a try:

<div v-for="(comment, idx) in comments" :key="idx">
  <!-- ... -->
</div>

The warning sources from <div v-for="comment in comments" :key="comment">, where the object comment is used as a key for v-for. The meaning of the warning is quite literal, don't use Object as key.

Use unique primitive value as key, maybe something like comment.id or ${comment.time}${comment.by}

You can simply avoid the use of :key in your v-for.

...
<div v-for="comment in comments">
...

As vuejs documentation said:

It is recommended to provide a key with v-for whenever possible, unless the iterated DOM content is simple, or you are intentionally relying on the default behavior for performance gains.