If you are making build for Android and you face error like --

Execution failed for task ':app:processDebugResources'.
> Failed to execute aapt

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

BUILD FAILED in 12s
    at ChildProcess.whenDone (/Users/deepika/desktop/fbLogin/platforms/android/cordova/node_modules/cordova-common/src/superspawn.js:169:23)
    at emitTwo (events.js:126:13)
    at ChildProcess.emit (events.js:214:7)
    at maybeClose (internal/child_process.js:925:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:209:5)
(node:2205) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:2205) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

it may be due to you haven't added FB Login app ID or app name in "<platform name="android"> </platform>"

you just have to

<platform name="android">
<config-file parent="/resources" target="./res/values/strings.xml">
            <string name="fb_app_id">YOUR APP ID</string>
            <string name="fb_app_name">Your APP NAME</string>        
</config-file>
</platform>

note- "YOUR APP ID" and "YOUR APP NAME" you will get from from facebook developer console

If you are making build for IOS and you face error like --

** BUILD FAILED **

The following build commands failed:

Ld build/emulator/myIonicAirWatch2.app/myIonicAirWatch2 normal x86_64

(1 failure)

(node:10536) UnhandledPromiseRejectionWarning: Error code 65 for command: xcodebuild with args: -xcconfig,/Users/max/Documents/cordova/myIonicAirWatch2/platforms/ios/cordova/build-debug.xcconfig,-workspace,myIonicAirWatch2.xcworkspace,-scheme,myIonicAirWatch2,-configuration,Debug,-sdk,iphonesimulator,-destination,platform=iOS Simulator,name=iPhone X,build,CONFIGURATION_BUILD_DIR=/Users/max/Documents/cordova/myIonicAirWatch2/platforms/ios/build/emulator,SHARED_PRECOMPS_DIR=/Users/max/Documents/cordova/myIonicAirWatch2/platforms/ios/build/sharedpch

(node:10536) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)

(node:10536) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

it mostly occur , when your using firebase , for my case i havent added the promise plugin. you can add it in  your project directory by typing command in terminal

$ npm install promise-polyfill --save-exact