Mailchimp Error: Bad Request - Your Campaign is not ready to send
While the "Your Campaign is not ready to send" message isn't very helpful, you can check for a more detailed message in MailChimp itself. Edit the draft that the API created, and navigate to the final Confirm step. You'll see a checklist where most of the items passed, but there will also be an item that explains why the campaign failed.
When I attempted to replicate the issue, the campaign failed to send because there was some default placeholder text left unchanged in the template. Since the code you posted only sets the content for one block, this is probably the same issue you're having.
Hope this helps!