When using a language that has accented characters (such as French, Spanish or German), it is not possible to type these characters with the onscreen keyboard if you are using the Touch Typing mode.
In versions of iOS prior to 11.0, you held your finger briefly on a character, until a pop-up appeared on screen showing any available alternate variants of the character. You would then slide your finger to the one that you desired. When you then lifted your finger, that variant of the character would be typed.
This is currently not working under iOS 11.0.x.