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.