What does it mean when you can't renew your book because the item has been requested?
This means that someone else is waiting to use that book and has placed an online hold on it via the Library portal. I suggest that you return it by the due date, then try to use it again after the requestor has finished with it.
Some high demand books that are on course unit reading lists will routinely be added to the Reserve collection. Please check with your local UNDA library whether or not there is an extra copy of the book, or whether it is available as an e-book.