Q: how do i copy a song from my macbook pro to my iPad?

https://discussions.apple.com/thread/7163823 - If you have signed up for Apple Music or iTunes Match you won't have the option to manually manage or sync music to a device in iTunes. Music syncing is done through iCloud and all your eligible music should appear there. Movies can still be synced.

A bit more about this, and some options- https://discussions.apple.com/thread/7194995 and: Move content from iTunes on your computer to your iPhone, iPad, or iPod touch

- https://support.apple.com/en-us/HT201593

and a workaround - https://discussions.apple.com/message/28842873#28842873

If you have not signed up for one of the iCloud based services:

https://discussions.apple.com/message/28481661#28481661 - "Music syncing has changed in iTunes 12.2.0 and iOS 8.4 Music App."