Hi Serge- Once you upload your book a copy must be ordered within 15 days. If not, our system automatically deletes it to make room on our server.
We cannot stop a book from being deleted because that's controlled by an automated process. But if the uploaded file is in danger of being deleted you can either purchase a copy or let it be deleted and upload the file again later. (Open the book in the program you used to make it and then upload it again. The book will still be there unless you deleted the original file from your computer).
If you have any additional questions about this, please open a support ticket here: https://support.blurb.com