Go into Settings, then Reaction buttons. Tap on Button Configurator and using your emoji keyboard, type zap and choose the lightning bolt.
Not sure if there's an easier or better way or why it doesn't have a zap button by default.
Is there not a Zap feature on #nostur iOS app nostr:npub1n0stur7q092gyverzc2wfc00e8egkrdnnqq3alhv7p072u89m5es5mk6h0 ?
Discussion
Tagging nostr:npub1n0sturny6w9zn2wwexju3m6asu7zh7jnv2jt2kx6tlmfhs7thq0qnflahe and nostr:npub1lrnvvs6z78s9yjqxxr38uyqkmn34lsaxznnqgd877j4z2qej3j5s09qnw5 maybe they have a better answer.
That’s the way to do it to enable zaps on notes. Otherwise you can only zap from people’s profiles.