Please provide more details about your Mac. You MAY have to convert your OLD iTunes music library to the new Music app library before you do anything else. Before you go further, do yourself a favor and do a BACKUP. ZIP/compress the following files of your current iTunes folder.
Backup and zip these files together:
iTunes Library.itl
iTunes Library.xml (this file is what Plex needs to import from)
iTunes Library Extras.itdb
iTunes Library Genius.itdb
Album Artwork (FOLDER)
Previous iTunes Libraries (FOLDER)
If your iTunes library folder isn’t super large in terms of GB storage, I suggest you duplicate the whole contents of the old iTunes folder on a total separate external hard drive for safe keeping.
The basic questions first are:
1 - What version of MacOS are you CURRENTLY running? Big Sur? Catalina? or older?
Go to the little Apple logo on the LEFT corner of the desktop and click on ABOUT THIS MAC.
You can type this info up here or just take a screenshot of it.
Open the Grab or Screenshot app and take a snapshot of that information and upload it.
iTunes is no longer installable on Catalina and Big Sur. The default app on Catalina and Big Sur is the new standalone Music app. iTunes has been broken up into FOUR separate apps on the new versions of MacOS: MUSIC app (used for music), TV app (used for TV Shows, Movies, and to import your own video content), BOOKS (used for Audiobooks and Books), and lastly PODCASTS app. There is a new file and folder structure that the new apps have also.
The Music app can however convert your old iTunes library into the new one for it to use. Everything should almost be the same once converted.
Instead of iTunes Library.itl as the OLD iTunes library filename, the NEW Music app uses Music Library as the file name. Note: you cannot just rename your old iTunes library as the new one and expect it to automatically work. The new Music app will just create a new file and import the important data from the old file instead, but regardless make sure you have backups of the old files.
One minor but important note of the transition is that iTunes Library.xml is no longer an option to be exported in any of the new apps. iTunes Library.xml is just a file of the data in the original iTunes Library.itl file and is what Plex and other apps use to use to import iTunes playlists and ratings, leaving the original files alone.
In order for this XML file to be recreated with the new standalone apps, you’ll have to create it manually each and every time you make edits to any of your libraries of any app. You can do this for free and on your own, but if you like to support development of apps there is an app that is fairly new that allows automatic creation every X hours.