Might as well add my 2 cents. I just installed xcode and react native, following this guide: https://dev.to/rob117/how-to-really-set-up-a-react-native-environment-on-mac-248h
Everything seemed to run as expected at first with react-native run-ios...I copied over some react-native files from another project, got some dependency errors, fixed them with npm install, and then I restarted my MacBook. After I booted up again, I ran react-native run-ios and got errors about not being able to launch the iPhone 6 simulator. I couldn't find a fix, so I decided to determine if it was my code or if it was xcode or what. So I went to my main projects directory and started a new react-native project, referencing the link I posted above. After everything installed, I did react-native run-ios again and boom, everything worked fine.
So then I closed xcode, the simulator and the terminal that runs upon starting react-native. I went back to my first project and ran react-native run-ios. boom it's working again, except this strange error error Could not find the following native modules: RNKeychain, RNVectorIcons. Did you forget to run "pod install" ?. I installed cocoapods via brew the first time around, I tried pod install but it complains about no xcode file or something. I've tried pod init, but it tells me No Xcode project found, please specify one. I ran pod setup, no error...seemed to do nothing? I guess it's working because now my simulator is running again.
Maybe I'll just remove the react-native-keychain and react-native-vectoricons npm packages from my package.json and see if it goes away. I don't really understand what coca pods is or why I need it. I'm thinking maybe you should take some time to update xcode, delete node_modules folder and run npm install again, spin up a new react-native project and if it works close xocde, simulator and the terminal and then try to go run your project again.
If it was working fine before and you didn't update anything outside of node_modules folder, remove and reinstall often works for me.
Good luck