keystore not found for signing config 'release'.-- React native

I'm trying to generate a Signed APK for react native project. Followed steps from android and react native documentation.

my Build.gradle file

android {
compileSdkVersion 23
buildToolsVersion '26.0.2'

defaultConfig {
    applicationId "com.appmobile"
    minSdkVersion 16
    targetSdkVersion 22
    versionCode 1
    versionName "1.0"
    ndk {
        abiFilters "armeabi-v7a", "x86"
    }
}
signingConfigs {
    release {
        if (project.hasProperty('MYAPP_RELEASE_STORE_FILE')) {
            storeFile file(MYAPP_RELEASE_STORE_FILE)
            storePassword MYAPP_RELEASE_STORE_PASSWORD
            keyAlias MYAPP_RELEASE_KEY_ALIAS
            keyPassword MYAPP_RELEASE_KEY_PASSWORD
        }
    }
}
buildTypes {
    release {
        signingConfig signingConfigs.release
        // minifyEnabled enableProguardInReleaseBuilds
        // proguardFiles getDefaultProguardFile("proguard-android.txt"), "proguard-rules.pro"
    }
}
splits {
    abi {
        reset()
        enable enableSeparateBuildPerCPUArchitecture
        universalApk false  // If true, also generate a universal APK
        include "armeabi-v7a", "x86"
    }
}
// buildTypes {
//     release {
//         minifyEnabled enableProguardInReleaseBuilds
//         proguardFiles getDefaultProguardFile("proguard-android.txt"), "proguard-rules.pro"
//     }
// }
// applicationVariants are e.g. debug, release
applicationVariants.all { variant ->
    variant.outputs.each { output ->
        // For each separate APK per architecture, set a unique version code as described here:
        // http://tools.android.com/tech-docs/new-build-system/user-guide/apk-splits
        def versionCodes = ["armeabi-v7a":1, "x86":2]
        def abi = output.getFilter(OutputFile.ABI)
        if (abi != null) {  // null for the universal-debug, universal-release variants
            output.versionCodeOverride =
                    versionCodes.get(abi) * 1048576 + defaultConfig.versionCode
        }
    }
}

}

I have placed keystore file in Android/app directory

my grade.properties

    android.useDeprecatedNdk=true
MYAPP_RELEASE_STORE_FILE=sampleReleasekeyStore.jks
MYAPP_RELEASE_KEY_ALIAS=*****
MYAPP_RELEASE_STORE_PASSWORD=*****
MYAPP_RELEASE_KEY_PASSWORD=*****

when executing cd android && ./gradlew assembleRelease.

Its throwing Build failed

Execution failed for task ':app:validateSigningRelease'. Keystore file /Users/username/Projects/appMobile/android/app/sampleReleasekeyStore.jks not found for signing config 'release'.

I'm using Mac environment. Please let me know where I'm going wrong.