Typically, this is due to the privacy settings on your personal Facebook page. If you have them set so that only your friends can see your posts, you won’t be able to share your posts on your Facebook Business page. Once the privacy of a post is set to Public, then it can be shared.
Keep in mind that, by default, you can share any post from your Business page on your personal page. This is a great way to reach followers in your personal network.
For more info, check out Facebook’s resource on the topic.