Apple recently changed how iTunes works and you now have to manually install your ringtones. What iTunes removed was the automatic management of ringtones. Now the only option is to manually install ringtones using iTunes. To do this you can follow the steps listed in this Apple help article: https://support.apple.com/en-us/HT207955.
I have included the relevant parts of Apple's article here:
Move ringtones from iTunes to your iPhone, iPad, or iPod touch
If you have a ringtone or tone in iTunes on your Mac or PC that you want to move to your iOS device, you can manually add it to your device using iTunes.
- Connect your device to your computer.
- Open iTunes. Make sure that you have the latest version.
- On your computer, find the ringtone or tone that you want to add to your device. If the ringtone or tone previously appeared in your iTunes library, you can find it in the Tones folder of your iTunes media folder.
- Select the tone that you want to add to your device and copy it. (You can use a keyboard shortcut.)
- Return to iTunes and go to the Library tab of Music.
- Click on your device in the left sidebar, then click Tones.
- Paste your tone. (You can use a keyboard shortcut.)