`react-native run-ios` returns Error: Could not find iPhone X simulator

Temporary fix:

Step 1: Open the file /node_modules/react-native/local-cli/runIOS/findMatchingSimulator.js

Step 2: Change a line of code, from this:

if (!version.startsWith('iOS') && !version.startsWith('tvOS')) {
  continue;
}

to this:

if (!version.startsWith('com.apple.CoreSimulator.SimRuntime.iOS') && !version.startsWith('com.apple.CoreSimulator.SimRuntime.tvOS')) {
  continue;
}

Step 3: Run react-native run-ios (you might need to run it twice, and don't forget to kill the Metro Bundler [the console that's running in the background] if it's running from your previous unsuccessful build!)

The problem was that after an update to Xcode - the simulator namespaces were added to the devices' versions list. The React Native build wasn't expecting these namespaces - hence the build break.

Edited: I copied your devices section of your output JSON and embedded to my own /node_modules/react-native/local-cli/runIOS/runIOS.js

function runOnSimulator(xcodeProject, args, scheme) {
return new Promise(resolve => {
    try {
        var simulators = {devices section of your json}; //Here
    } catch (e) {
        console.log("e", e);
        throw new Error('Could not parse the simulator list output');
    }

    const selectedSimulator = findMatchingSimulator(simulators, args.simulator);
    console.log("selected", selectedSimulator);
    if (!selectedSimulator) {
        throw new Error(`Could not find ${args.simulator} simulator`);
    }
    ...

And finally, it gave the same error as yours. So I figured out that parsing version of devices different. In your devices, version is;

"com.apple.CoreSimulator.SimRuntime.tvOS-12-1" //for tvOS
"com.apple.CoreSimulator.SimRuntime.iOS-12-1" // for iOS

but in react-native checks this version values like this(/node_modules/react-native/local-cli/runIOS/findMatchingSimulator.js);

// Making sure the version of the simulator is an iOS or tvOS (Removes Apple Watch, etc)
if (!version.startsWith('iOS') && !version.startsWith('tvOS')) {
    continue;
}

So react-native can not recognize.
If we change this code with this;

// Making sure the version of the simulator is an iOS or tvOS (Removes Apple Watch, etc)
if (!version.startsWith('com.apple.CoreSimulator.SimRuntime.iOS') && !version.startsWith('com.apple.CoreSimulator.SimRuntime.tvOS')) {
    continue;
}

the problem was solved. I checked that with your JSON data in my computer and it worked.

To fix this error, simply reinstall project & node modules using npm

npm install

Then it will show a list of warnings, and to fix them use the following command:

npm audit fix

This should fix everything, and allow you to run iOS emulator react-native run-ios

PROBLEM

If you try this with the latest Xcode (11), there is no iPhone X!

Run Simulator by itself, in the top menu, look under Hardware, Device, iOS 13.0. You will see there is:

• iPhone 8
• iPhone 8 Plus
• iPhone XS
• iPhone XS Max
• iPhone XR
• ... and some iPads

When you execute run-ios, react-native is designed to match a requested device.

The internally hard coded default is iPhone X.

The function that tries to match the requested device is in:

/node_modules/@react-native-community/cli-platform-ios/build/commands/runIOS/findMatchingSimulator.js

This function is designed so you can give it a device, and an optional version number.

If the given device and version cannot be found, it will return a match using the first device in the list by default.

But... in reality, the first device is a watch, and any watch is excluded from matching, so this function will return null.

SOLUTION 1 - Use an existing Xcode device

Run Simulator first by itself, as described above, and make a note of which iPhone or iPad you want.

Then pass this name as an optional argument the the run-ios command line command as follows:

react-native run-ios --simulator="iPhone 8"

SOLUTION 2 - Add New Xcode iOS Device

According to Xcode 11 Release Notes:

"Xcode no longer creates every available iOS simulator device by default. Instead a set of the most commonly used devices are created. To create other devices — or multiple instances of a device — open the Devices window, select Simulators, click the + button, enter a name, and select the relevant device type and OS version. In Terminal, execute the xcrun simctl create command, for example xcrun simctl create "My iPhone 7" "iPhone 7" iOS13.0. (49428617)"

In Xcode, you need to add a new device called "iPhone X".

Also I answered here: React Native Issue #2328