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 10 playlists per 24 hours. 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 !
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 !