In case you are talking about the Ryze Tello app:
de-install, reboot, re-install, re-grant permissions. This _could_ fix the problem.
Check if there is an iOS update. If so, install and re-do the previous procedure. This again _could_ fix it.
In case you are talking about the TelloFpv app:
go into video settings and uncheck "use TelloFpv album". This makes the app use an alternative but outdated and deprecated way of adding to the photo roll, which for some reason works when the documented one fails.
I found the hard way that the photo roll gallery can be a real bi t ch. What works on 99,x% of all phones fails on a handful, with no indication why. Nor related to the Iphone model, and not related to the iOS model. Must be some strange combination that fails.