That message has been appearing for at least three months. Sometimes it means nothing and can be ignored. You'll know if everything is okay if your ebook is available for sale on B&N's website within 72 hours.
Sometimes the message is real and you'll have to try uploading your ebook again later. But there's no way to tell until you wait a while to see if your first upload was received okay and published.
What you should avoid is uploading your book again until you know for certain that the first time didn't work. Otherwise, you may wind up with multiple copies of the same ebook for sale.
Just keep checking B&N for your ebook for the next three days. Hopefully it will appear soon.
Kind regards, David