Hey, y'all. I'm having this same issue. See screenshot here. My video clip does not have a native audio track; I recorded video only from my iPhone (FilmicPro). And I recorded audio separately. Will Premier not let you sync audio if you didn't originally have native audio? I'm a newbie, so might have just completely made the mistake of not recording native audio to be able to use this feature.
It has to have audio to sync to ... so if there's no audio with the video, it can't sync to anything. So for that pairing, you'll need to visually sync things. It's often better to set the timeline to display audio units when sliding audio to get a visual match ... right-click the 3-bar menu to right of sequence name on timline panel, then select "show audio units".
Get you audio matching the mouth, then go back to frames or whateve you prefer.