You must turn off double optin inside AWeber before you can use the plugin.
Unfortunately, for AWeber users Confirmed opt-in is not configurable from the API - this is something that clients will have to contact AWeber for on a case-by-case basis.
So you must contact AWeber support so they turn off the double optin for the API for you. It's not going to work if you disable double optin in your List Settings. It has to be turned off for the API.
There's a help article with a little more information here: