In order to enable single-opt-in for Aweber when emails are being added through the API (by a 3rd party product like UpViral), Aweber needs to manually approve your account to do that. 

So, if you want to have your account and your lists enabled for this, all you have to do is contact AWeber through their online form, or email help@aweber.com.

Tell them you want to enable single opt-in for your lists for use through their API. AWeber will then either approve or deny your request after they review your account.