Cannot find the first song from an album I bought in 2015. How do I get it back without purchasing the album again, which is what it is asking me to do?

Hi there!

You may want to check if it's been hidden.

Hide and unhide music, movies, TV shows, audiobooks, and books - Apple Support

Unhiding it will allow you to redownload the song

If you still can't redownload it withput getting charged, most likely this song has already been version has already been removed by the content provider and have replaced it with a newer version that can work with the current software versions of Apple devices.

You can always Contact - Official Apple Support for further help

Will,

If you are still using the same Apple ID, are in the same country, and the content is still available, you may be able to redownload fresh copies at no charge. Follow the steps in here:

Redownload music - Apple Support