It is not downloading the song once i hot download all

When you buy a song, you can't use that song as a ringtone, unless you first make that song suitable to use as a 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