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 up to this limit. Be careful, this limit is for the whole Google account connected to YouTube AND YouTube Music.
Here is a forum discussion about this new limitation if you want to know more.