Once you have installed and enabled Microsoft SwiftKey and given it Full Access, you may still need to select Microsoft SwiftKey as your keyboard the next time you type.
If a keyboard other than Microsoft SwiftKey appears, tap the globe icon to the left of the spacebar to navigate through your installed keyboards. Microsoft SwiftKey will remain as the default keyboard until you manually switch to another.
*Please note that some apps do not support the use of third party keyboards, so if Microsoft SwiftKey is not showing up, this is likely because the app creator or Apple has made requirements to only use the default keyboard on your device.*