I have an iPhone 6 and I have 64 GB of storage, why doesn't Fortnite work?
Because of the performance of the iPhone 6.
Only the iPhone 6S is officially supported, you can't do anything.
Fortnite requires at least 2 GB of RAM on iOS, but the iPhone 6 has only 1 GB.
PUBG might be an alternative for you. When I tried it a while ago, it was halfway playable even on the iPhone 5s (it was enough for a chicken dinner…)
In terms of performance, it doesn't even fail - after all, the game also runs on the iPad mini 4. But 1 GB RAM is just not enough for Fortnite.
But if the ram is missing… Then that is also part of my performance.
Seen this way… Well.