unable to find a song from apple music in the iTunes store

Another possibility is that the song is in the Store but -- the search function being wonky -- did not turn up on your search. Try various searches in the Store on artist, title, album, keywords before you give up.

macgyver too wrote:

I found a song in Apple Music which I wanted to buy, but unable to find it in the iTunes Store.

If it's not in the iTunes Store, you can't buy it there. It's possible that the rights holder of the song are only offering it for streaming but not for sale.

MacGyver Too,

It may be available for purchase from another music download retailer such as Amazon, Bandcamp, Beatport, etc.

If you wish to share what song it is, perhaps someone here can help.