[Koha] help! - Enable place hold in the opac.

Hello, I am a college student who has just joined the mail-list. I give my apologies in advance for my English, I am an Italian student. I'm learning to use Koha reading the manual and found myself in a big problem in the OPAC. I do not understand why I can not activate in the OPAC the place hold especially in my cart.How i can enable this feature?I added a few books in the catalog and some users, I use Koha 3.0.4. I need your help.