Download Itune music (song) purchased

1.

It shows up as purchased on the Itunes app

Then check your Music Library -> Recently added.

Or try to redownload, see -> Redownload music -> Redownload music - Apple Support

2.

I wanted to use it for my ringtone

To make and/or use a song as a ringtone, you must first make that song suitable for a ringtone.

See the following Apple articles:

Create and manage ringtones

Create and manage ringtones - Apple Community

Create a ringtone of your Logic Pro or GarageBand for Mac song

Create a ringtone of your Logic Pro or GarageBand for Mac song – Apple Support (UK)

Create Ringtones in Garageband

Create Ringtones in Garageband - Apple Community