The key goes on the element the map returns
The warning says a key is missing, you add one to the inner element, the warning stays. React needs the key on whatever the callback returns, which is the outermost element, fragment included.
{posts.map((post) => (
<Fragment key={post.slug}>
<Separator />
<PostCard post={post} />
</Fragment>
))}<> cannot take a key. Import Fragment when you need one.
react