How to turn off autocorrect for iPhone 6 Mail?
Make sure you’re using the apple keyboard.
Since keyboard setting changes made in the Settings apps only apply to be built-in Apple keyboard, you’ll need to make sure that you’re using it. Touch and hold the globe key on your keyboard to change back to your Apple localized keyboard.
You could also disable autocorrect in the companion app for your custom keyboard, in your case the SwiftKey app.