When you restore your iTunes library from a backup, the media files themselves are not copied over. Instead, iTunes creates links to the files in their original location. If you have moved or deleted the files since you created the backup, iTunes will not be able to find them and will not be able to add them to your library.
To check your iTunes media folder, open iTunes and go to Preferences > Advanced. The "iTunes Media Folder Location" field will show you the location of your iTunes media folder. Open the folder and make sure that the files you are trying to add to your library are present.
2. Re-import the files.
If the files you are trying to add to your library are not present in your iTunes media folder, you will need to re-import them. To do this, simply drag and drop the files from your computer into the iTunes window. iTunes will automatically add the files to your library.
3. Authorize your computer.
If you are trying to add music that you purchased from the iTunes Store, you may need to authorize your computer in order to play the files. To do this, open iTunes and go to Account > Authorizations > Authorize This Computer. Enter your Apple ID and password when prompted.
4. Contact Apple Support.
If you have tried all of the above steps and still cannot add the music to your library, you may need to contact Apple Support for help.