The help section says to go to settings but all I see is an option for language, nothing for sync or “offline.”
See this post from the official Google support site. It has a detailed explanation of how to setup offline access to Drive for your Chromebook.
Enabling Google Drive offline
Open Google Drive either by clicking the Drive icon or by going to drive.google.com while you are online.The sync process will begin automatically.
Look for a notification at the lower right corner of your screen to see how many of your documents have synced.Remember that you must have at least one saved document stored in Google Drive to use offline.
After Google Drive is finished syncing, you can access your documents offline. Any edits you make to documents while you are offline will be synced the next time you’re online.