Technical Details
App crashes on launch
(Assuming you've installed it with AltStore)
AltStore can “code sign” apps for a week, and they should be resigned before that time runs out. However, if you're away from your computer (AltServer) or the connection hasn't worked for some reason, the app might become unsigned and fail to launch because of that; iOS will run only apps with a valid signature.
No data has been lost, unless you've deleted the app manually. Simply open the AltStore app again on your device and verify that the resigning succeeds, and Tails will return to normal. If you can't open AltStore either, reinstall it first with your computer.
Deleting Tails will delete all data associated with it, most importantly your favorited tags. Do not delete the app as a first measure if you're trying to fix code signing issues.
How do I install AltStore?
There are easy to follow instructions on AltStore's FAQ page for both Windows and Mac. AltStore doesn't currently support Linux, but it has been discussed.
Tip: Since AltStore (and other such tools) require the use of an Apple ID, I recommend creating a new one, that you only use for these tools.
I don't want to install AltStore
AltStore seems like the most frictionless solution to me, and that's the only installation method I've personally tested and verified to work, but you can always sideload the .ipa file with any sideloading tool of your choice. Just be sure to always download the .ipa file only from the official source, https://tails.safo.fi.
In the future you might come across some 3rd party cloud services offering to install Tails without a computer. These services always require you to trust a company to install an enterprise certificate or a profile onto your device. These might be “Mobile Device Management” (MDM) profiles, meant for system administrators of schools or large businesses. MDM allows access for a lot of settings and options, including clearing your device's passcode or erasing your device completely. In the past MDM profiles have also been abused to track user location, app use, email accounts, camera permissions, and browsing history.
Even if a certificate management service isn't installing an MDM profile on your device, the .ipa file they install might be a modified version of the official release. These modifications might be unnoticeable and involve only some sort of user tracking, but it wouldn't be impossible to exploit any vulnerabilities in iOS to again gain access to your entire device.1
When installed from an unofficial source, ads could also be injected into the app – if you see any ads in Tails, you're definitely using an unofficial build. By using any version of Tails with ads, you're only benefitting the party that has modified the .ipa; you're not supporting the development of this app, not the upkeep of e621, and not any artists.
Why is iOS 14 required?
iOS 14 introduced a lot of quality of life improvements for app developers, and some bug fixes that I'd previously needed to work around. To the user these improvements might result in better performance in certain screens of the app, but honestly, it's just nicer to write modern code for modern frameworks. I expect to be adopting new programming features pretty aggressively in the future, too.
In case you don't want to update to iOS 14, you could view older releases and download a 1.x release for iOS 13, but note that iOS 14 can be installed on all of the same devices that ran iOS 13.
Will there be an Android version?
An Android version of Tails is unlikely to happen – Tails wasn't written as a cross-platform application. It's heavily based on iOS' UI frameworks and written entirely on Apple's Swift programming language.