The in_category function works only on existing (saved) posts. Here you can directly access the category in $postarr like this:
if ('affitti' == $postarr['post_category'])) {
wp_die('Error: Posts cannot be published with the default category.');
}
By the way, are you sure you want to use wp_die for this and not some other error message? People may loose their work if WP stops completely when they try to publish.
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