[Koha] trouble placing hold on book

I tried and failed to place a hold on a book today. The book was already checked out and I wanted to place a hold so that when it was checked in, our volunteer would be alerted. I went to the book record, clicked place hold, then entered the student's number. I received the message "no copies are available to be placed on hold". Well of course not, if there was a copy available, I wouldn't need to place a hold - lol. Why can't I place the hold? Thanks, Shelley