Keystore file not set for signing config release flutter (when trying release mode)

Hey there I need your help. I know there are many threads with this error, but I tried every single solution and nothing is working.

I did everything like it's mention in the documentation to get my app bundle, but I always get this error:

Execution failed for task ':app:validateSigningRelease'.
> Keystore file not set for signing config release 

I made the key.properties like this and saved it in the android folder (C:\Users\Martin\Documents\tutorial\android):

storePassword=XXXXXX
keyPassword=XXXXXX
keyAlias=key
storeFile=/Users/Martin/key.jks

I created my own keystore file which is located in /Users/Martin/

And finally I changed my build.gradle as mentioned in the documentation.

def localProperties = new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {
    localPropertiesFile.withReader('UTF-8') { reader ->
        localProperties.load(reader)
    }
}

def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}

def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
    flutterVersionCode = '1'
}

def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
    flutterVersionName = '1.0'
}

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'
apply plugin: 'kotlin-android'
apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"

   def keystoreProperties = new Properties()
   def keystorePropertiesFile = rootProject.file('app/key.properties')
   if (keystorePropertiesFile.exists()) {
       keystoreProperties.load(new FileInputStream(keystorePropertiesFile))
   }




android {
    compileSdkVersion 29

    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
    }

    lintOptions {
        disable 'InvalidPackage'
    }

    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "com.example.myID"
        minSdkVersion 16
        targetSdkVersion 29
        multiDexEnabled true
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
    }


   signingConfigs {
       release {
           keyAlias keystoreProperties['keyAlias']
           keyPassword keystoreProperties['keyPassword']
           storeFile keystoreProperties['storeFile'] ? file(keystoreProperties['storeFile']) : null
           storePassword keystoreProperties['storePassword']
       }
   }
   buildTypes {
       release {
           signingConfig signingConfigs.release
       }
   }

}

flutter {
    source '../..'
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'com.android.support:multidex:1.0.3'
    implementation platform('com.google.firebase:firebase-bom:26.1.1')
    implementation 'com.google.firebase:firebase-auth'

}

I don't know if this is helpful, but if I try the debug mode

   buildTypes {
       release {
           signingConfig signingConfigs.debug
       }
   }

it works just fine and everything works.

√ Built build\app\outputs\bundle\release\app-release.aab (20.0MB).

What could cause this error in release mode but works in debug mode? Thanks for the help.