I posted about this earlier in another thread.
With the new UI you must use up the free trial 25km first (a pop-up will appear during your ride saying your trial is over but you can close that pop-up and finish your current ride going beyond the free 25km). After the free trial is over pop-up the next time you load Zwift you will get a pop-up on the home screen and only then will the Restore Purchases button be available (screenshot is in the link).