Thanks for visiting the Apple Support Communities.
It seems like you purchased a ringtone but you can't use it on your iPhone. Let's look into this together.
Did you previously purchase the song from the iTunes Store? If so, you shouldn't need to worry about accidentally buying it again.
To verify what was purchased, you can use the steps in
See your purchase history in the App Store or iTunes Store
Redownload music, movies, TV shows, apps, and books from the iTunes Store, iBooks Store, and App Store.
Make sure you're following these instructions to use ringtones:
Use tones and ringtones on your iPhone, iPad, or iPod touch - Apple Support
If you bought a ringtone but it's not on your iPhone:
- On your iOS device, go to Settings > Sounds & Haptics.
- Tap the sound that you want to change. You can change your Ringtone, Text Tone, New Mail sound, Calendar Alerts, and more.
- Tap on the name of a ringtone or alert tone to hear it play and set it as the new sound.
You can also set a Ringtone or Text Tone for a specific contact. Find the person in Contacts, then tap Edit in the upper-right corner. Tap Ringtone or Text Tone, then choose a new sound.
With iOS 11, you can redownload purchased tones that you bought using your Apple ID.
Go to Settings > Sounds & Haptics, then tap on any sound. At the top of the page, tap Download All Purchased Tones to check for any previously purchased tones not currently downloaded to your device. You might not see this option if you've already redownloaded your purchased tones or if you haven't purchased any tones.
You might not be able to redownload some previously purchased tones.