The topic_lookup_from_imported_post_id() method returns a hash.
So, I’d rewrite your function to something like this:
create_posts(posts, total: total) do |t|
parent = topic_lookup_from_imported_post_id(t["Thread_ID"])
next unless parent
source_user_id = @usernamesMap[t["username"]]
opts = {
id: t['id'],
user_id: user_id_from_imported_user_id(source_user_id) || Discourse::SYSTEM_USER_ID,
raw: t["Post_Description"],
created_at: t["created_at"],
title: t["title"],
topic_id: parent[:topic_id],
reply_to_post_number: parent[:post_number]
}
end