To create a playlist under your YouTube or YouTube Music account, Soundiiz use the official API made by Google for YouTube Datas.
Unfortunately, recently Google set a new limit to the number of playlists we can create with the API...
No official documentation, but it seems to be around 10 playlists per 24 hours (not an exact value, could be less; could be more). Once you are reaching the limit you will get on Soundiiz a "307" code Error message (or "304"). You have to wait at least 24 hours before retrying.
Once this delay past, you will be able to convert again playlists to YouTube with Soundiiz up to this limit. The limit is against the developer account using the YouTube Data API. You can still create playlists manually directly on YouTube and YouTube Music website.
Here is a forum discussion about this new limitation if you want to know more.
Also, if you are getting an error like "internal error" or something not specified, it's probably that we have reached YouTube API Quota limit. Try again later, that should be good again as the Quota is reset once a day !
I never transferred a playlist and got this error message
It's looks like you are not using the right YouTube account here (be sure to use a "YouTube Channel" - it's required to create playlists, even for YouTube Music).
To be sure to use the good account/channel when connecting YouTube / YouTube Music to Soundiiz, create a playlist on your YouTube / YouTube Music application with a name like "Test YouTube Soundiiz".
Then on Soundiiz reload the page and as long you are not seeing this playlist listed on Soundiiz "Playlists" tab, that means you are not connected with the right channel.
Disconnect on Soundiiz and try to connect it back by this time connecting first the Google account and then picking the right YouTube channel.
You can still create an empty new playlist on YouTube / YouTube Music directly on your mobile app or website YouTube. Once done, go back to Soundiiz and reload the interface. The recent playlist created should be listed there.
Now select the source playlist you want to convert to YouTube / YouTube Music and in the option ("..." or right click) of the playlist, choose "Add to playlist". Simply select your recent empty created playlist and start the process !
Another workaround is to move first from your source (like Spotify) to Google Play Music with Soundiiz. Once done and your datas are under Google Play Music, use the official migration flow provided by Google to do GPM => YouTube Music.