Unable to sync song in iTunes?

My parents' laptop stopped working. I created a secondary account for them in my MacBook Pro and logged in with that. When I opened up iTunes, I connected my father's iPhone and tried to sync a song I downloaded somewhere else. I don't know what the problem is, but since this computer has my Apple ID and such, and I'm the administrator, I'm guessing that could be the issue. Also, every song transferred from his phone to iTunes has the iCloud logo, except the song I downloaded, which.

"""computer has my Apple ID… I'm guessing that could be the issue"""

Great guess! Log out of the Store and log in using the same Apple ID that bought the song. Then click the "Store" menu again, and choose "Authorize this computer". Your parents are not permitted to distribute purchased music, except for use by them (their Apple ID is "them") on up to five computers. Your Apple ID is not permitted to "borrow" anyone else's music, even that purchased by blood relatives.