It’s possible that your chosen domain name has already been requested by another person and is pending registration with the .it Registry.
In this case you need to wait for the response from the .it Registry: if the domain name is assigned to the other person, you may request from Aruba.it the same domain name with a different extension (.com .net .org, etc), if available, or a different domain name altogether.
In any case you must place the order for the new domain name (without making a new payment) and send a request to use the credit .
To check the registration status of .it domains simply go to: http://www.nic.it/web-whois and enter the domain name in full without the www (e.g. aruba.it).