If you can't add a new address to the whitelist, make sure the address is relevant for the cryptocurrency.
Then check, if attempting to set up an address on the Bitcoin network, that the address is not Segwit (Segregated Witness). At the moment XGo doesn't support Segwit addresses but this will be updated soon.
To add an address to the whitelist you'll need to confirm this action via email containing a link which is active for 10 minutes
Attention: If you ever receive an email with a confirmation link for any action related to Whitelist, and you haven't recently initiated this, the most possible explanation is that your account has been compromised.
However, with Whitelist enabled, you have at least 48 hours to get in touch via chat.