Hey X_yan,
Welcome to the Apple Support Communities! I see you're unable to download a digital booklet that came with an album you purchased. I can appreciate why you'd be concerned about that, so I wanted to reach out with a few suggestions that could help. The way you're supposed to access them is through iTunes (on your computer exclusively), select Library at the top of the screen, followed by Songs. Here's more on that: Redownload apps, music, movies, TV shows, and books from the App Store, iTunes Store, and Apple Books - Apple Support
If you aren't seeing it there, follow these steps to make sure it's not a hidden purchase: Hide and unhide music, movies, TV shows, audiobooks, and books - Apple Support. If unhiding purchases doesn't yield the download, double-check that you did purchase the version that includes the digital booklet: See your purchase history in the App Store or iTunes Store - Apple Support
If you've confirmed through the purchase history that you should be receiving a digital booklet and you're still unable to locate it, you'll want to reach out to iTunes Support directly using this link: Apple - Support - Topic Selection
Cheers!