There could be a number of reasons as to why you are unable to withdraw, below are some of the common causes and how it can be resolved:
This can be because you have "AddressBook Withdraw Only" checked in your security settings (https://www.cryptopia.co.nz/Security)
Uncheck "AddressBook Withdraw Only" and you will now be able to enter any address instead of your stored address book entries.
In the bottom section of the security you can add addresses for coins for convenience and security. (Use with Settings 2FA, to prevent unauthorized additions)
When you have added an address they will stored and available in the drop down menu on the withdrawal for that coin.