For some reason the per-lesson Tips button is missing on mobile (android) in the lesson selection screen, it has only the Key button. I see both buttons on desktop/browser version.
P.S. sorry for posting in a wrong thread, I don't know where is the appropriate one.
For all courses in Duolingo, the Tips & Notes created by volunteer contributors are only available on a web browser. You can probably see them by signing in to Duolingo on the browser of your mobile device, but they will be harder to read on the smaller screen.
Some courses do have Tips available on the apps. These are created by paid staff and are only available on the "major" languages. Staff has talked about making the system for Tips on the apps available for volunteer contributors, but is still working on how that would happen and do not currently have any time schedule for when that might happen.