Q: i had an error on one song while downloading an album how can i get that song

It may be a corrupt / damaged download.

If you live in a Region that allows re-downloading Music...

Delete the Song(s) and re-download...

See Here  >  Download Past Purchases  >  http://support.apple.com/kb/HT2519

If not... Contact iTunes Customer Service  >  Apple  Support  iTunes Store  Contact Us