This article explains two ways to download music to iPad from a computer using iTunes 12.6 and earlier: by choosing a category or by selecting individual songs.
How to Transfer Music from Your Computer to Your iPad, iPad mini, iPad Air, and iPad Pro
Syncing music, movies, apps, and other content to your iPad can be as simple as plugging a cable into the dock connector on the bottom of your iPad and the other end into a USB port on your computer and letting it download. For more control over downloading music to your iPad, familiarize yourself with the music syncing options in iTunes.
Connect your devices using the cable that came with your iPad and open iTunes.
Tap the iPad icon at the top of the screen.
