Your Participants' iOS device will have to be on one these OS Versions to download Lookback's iOS Participate App:

✅ iOS 16.0, 16.1, 16.2, 16.3, 16.4

✅ iOS 15.0, 15.1, 15.2, 15.3, 15.4, 15.5

✅ iOS 14.0, 14.01, 14.1, 14.2, 14.2.1, 14.3, 14.4, 14.5, 14.6, 14.7, 14.8

✅ iOS 13.7

✅ iOS 13.6. 13.6.1

✅ iOS 13.5, 13.5.1

✅ iOS 13.4, iOS 13.4.1

✅ iOS 13.4, iOS 13.4.1
✅ iOS 13.3, iOS 13.3.1
✅ iOS 13.2, iOS 13.2.1, iOS 13.2.2, iOS 13.2.3
✅ iOS 13.1, iOS 13.1.1, iOS 13.1.2, iOS 13.1.3
⚠️ iOS 13.0
✅ iOS 12.4, iOS 12.4.1, iOS 12.4.2, iOS 12.4.3, iOS 12.4.4, iOS 12.4.5, iOS 12.4.6, iOS 12.4.7
✅ iOS 12.3 , iOS 12.3.1, iOS 12.3.2
✅ iOS 12.2

⚠️Oops iOS 13.0 was released with some bugs that prevent Participate from working!
If you or your participants are on iOS 13.0 please request them to update
(To update, tap: Settings > General > Software Update > Download and Install)

✅ iPadOS 16.0, 16.1, 16.2, 16.3, 16.4

✅ iPadOS 15.0, 15.1, 15.2, 15.3, 15.4, 15.5

✅ iPadOS 14.0, 14.01, 14.1, 14.2, 14.3, 14.4, 14.5, 14.6, 14.7, 14.8

✅ iPadOS 13.7

✅ iPadOS 13.6

✅ iPadOS 13.5, 13.5.1

✅ iPadOS 13.4, iPadOS 13.4.1

✅ iPadOS 13.3, iPadOS 13.3.1
✅ iPadOS 13.2, iPadOS 13.2.2, iPadOS 13.2.3
✅ iPadOS 13.1, iPadOS 13.1.1, iPadOS 13.1.2, iPadOS 13.1.3

iOS 12.2 to iOS 13.3 and iPadOS13.1 to iPadOS13.3 Operating Systems (and Lookback's Participate App) are supported by these devices:

iPhone

iPad 

So if you're using one of these devices and your iOS update is indeed up-to-date, you're good to go. You can start testing on Lookback's iOS Participate App.

Pick up your device and tap:
Settings > General > Software Update > Download and Install

(It may take longer than a jiffy)