Quick answer
Soundiiz cannot display, transfer, export, or synchronize your YouTube Watch Later playlist or Watch History. YouTube does not make the contents of these private lists available through its official API. This limitation applies even when the correct Google account and YouTube channel are connected.
Why can't Soundiiz access these lists?
Soundiiz reads your YouTube playlists through the official YouTube Data API. Since September 12, 2016, requests for Watch Later and Watch History have returned empty lists. Google still documents both lists as unavailable through the API.
This is a YouTube API limitation. It is not caused by your Soundiiz plan, account connection, or a temporary transfer error. Reconnecting YouTube or upgrading to Premium will not make these lists available.
What can I do with my Watch Later videos?
In YouTube, save the videos you want to transfer to a regular playlist. Once YouTube makes that playlist available through its API, it can appear in Soundiiz and be transferred, exported, or synchronized like any other accessible playlist.
Can Soundiiz transfer my Watch History?
No. YouTube does not provide automatic API access to Watch History. Save the videos you want to keep to a regular YouTube playlist before using Soundiiz.
Official YouTube documentation
See the YouTube Data API revision history for the original September 2016 change and the current playlistItems.list documentation for the continuing limitation.