Effective September 2019, Osmo has stopped selling the iPhone base and reflector due to low interest in this device and the desire to offer customers the best possible game experience on other platforms.
That said, the Osmo games currently downloaded on your iPhone will work just fine! All you need to do is press the 'I understand, I still want to play' button to continue. New Osmo games released going forward will not be compatible, but again, you can continue to play the Osmo games currently on your iPhone indefinitely.
Osmo is compatible with these iPhones only:
iPhone 6, 6 Plus, 6s, 6s Plus, 7, 7 Plus, 8 and 8 Plus
Complete device compatibility list for reference: