Can't rename playlists in iTunes using Catalina OS.
Assume this is a bug that needs to be addressed in Catalina.
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First thing first, the app is now called Music and not iTunes! Renaming a playlist has been changed. Control click the playlist name and highlight "Open in New Window" from the drop-down menu. When the new window is displayed, click the top line, which is the current playlist name, and change it to whatever you want. I assume this is a work-around since nothing happens when double-clicking the playlist name.