Thanks for reaching out, skua123.
It sounds like you're trying to sync your music with your iPhone using iTunes. We can help.
It used to be that iTunes would allow you to create backups, restore your devices, and sync your iTunes library with your other devices. However, in recent versions of macOS this functionality was removed and is now handled by the Finder app, Apple Music, or through iTunes Match.
You can check out these steps from Turn on Sync Library with Apple Music:
"With the latest version of iOS, iPadOS, and macOS, Sync Library is turned on by default. If you turned off Sync Library, you can turn it back on. Just follow the steps below for your device.
On your iPhone, iPad, or iPod touch:
Go to Settings > Music.Turn on Sync Library. If you don't subscribe to Apple Music or iTunes Match, you won’t see an option to turn on Sync Library.
If Sync Library is updating or turned off, you'll see a message at the top of the Library tab in the Apple Music app.
On your Mac:
Open the Apple Music app.From the menu bar at the top of your screen, choose Music > Preferences.Go to the General tab and select Sync Library to turn it on. If you don't subscribe to Apple Music or iTunes Match, you won’t see an option to turn on Sync Library.Click OK."
Hope that helps!
Take care.