i purchased the song but i am unable to set it as ringtone

There's a difference between a song and ringtone. If you purchase an entire song, you can't set it as a ringtone without using an app like Garage Band to copy a portion of the song you want as a ringtone. Instead of buying the song, you should have looked in the iTunes Store for the Ringtone version of the song.

sukhmani33 wrote:

i see,but i got no option asking me to apply the song for ringtone when i opened it from the settings on set ringtone from i tune

That's because you searched for the song, not the ringtone. For example, I can search the iTunes Store for Mama Mia from ABBA and buy the song. Or I can go into the iTunes Store and search for Mamma Mia Ringtone and be directed to the ringtone version of the song.

It's been this way forever on iPhone. You buy a Ringtone. You can a song. A song cannot just be selected as a Ringtone unless you use something to convert the portion of the song you want to use as a Ringtone.

If you bought the full length song, refer to this article Create a ringtone of your Logic Pro or GarageBand for Mac song - Apple Support

If you have the piece as a ringtone, read Use tones and ringtones with your iPhone or iPad - Apple Support

Is that new that we can’t use songs for ringtones? Because I have, like, 10-ish songs that I’ve been using as ringtones for a long time in my list, whether I go through Settings/Sounds or assigning one to a specific contact. But I can’t do it today.

Not on an iPhone, are you sure you weren't using an Android device?