Fortnite iOS players updating to iOS 14 may lose access to Fortnite. If the message “Temporarily Remove Apps to Install the Software Update?” is accepted, it may result in Fortnite being deleted. Fortnite cannot later be reinstalled due to Apple preventing users from doing so.
iOS players who wish to continue playing Fortnite should press Cancel first. Then, to free up space you can offload apps or media by going to Settings->General->iPhone Storage. Once you have cleared out enough space, attempt the update again and the prompt will no longer appear.