To test the mobile application you follow these steps from BUILD.md:
Testing the Mobile application
First you need to setup React Native to build projects with native code. For this, follow the instructions on the Get Started tutorial, in the “React Native CLI Quickstart” tab.
Then:
cd ReactNativeClient
npm start-android
# Or: npm start-ios
To run the iOS application, it might be easier to open the file ios/Joplin.xcworkspace on XCode and run the app from there.
Normally the bundler should start automatically with the application. If it doesn’t run npm start .
Have you tried this?