Gradle SubProject tasks are not getting picked-up

I have a multi-module setup for a Java project with following structure.

mainApp
|--> core-module
|       |--> src
|       |--> build.gradle
|       |--> gradle.properties
|       
|--> lib-module
|       |--> src
|       |--> build.gradle
|       |--> gradle.properties
|--> lib-another-module
|       |--> src
|       |--> build.gradle
|       |--> gradle.properties
|--> settings.gradle
|--> build.gradle

in mainApp/build.gradle I've mentioned

subprojects {
    test.dependsOn "CreateMessageKeys"
    //test.dependsOn ":CreateMessageKeys"
    //test.dependsOn ("CreateMessageKeys")
    //test.dependsOn (":CreateMessageKeys") none of this working....

}

task CreateMessageKeys(type: CreateMessageKeysTask) {
    destDir = "bundle-common/src/";
    outputClass = "common.messages.MessageKeys";
}

and my core-module/build.gradle have a test target as

test {
    useTestNG() { 
        useDefaultListeners = true
        suites 'test/testng.xml' 
    }
}

but getting error as below.. What Am I missing here?

Caused by: groovy.lang.MissingMethodException: No signature of method: java.lang.String.dependsOn() is applicable for argument types: (String) values: [CreateMessageKeys]
Possible solutions: respondsTo(java.lang.String)

Edit

subprojects {
    test.dependsOn(rootProject.tasks['CreateMessageKeys'])
}

task CreateMessageKeys(type: CreateMessageKeysTask) {
    destDir = "bundle-common/src/";
    outputClass = "common.messages.MessageKeys";
}

It generates the error:

* What went wrong:
A problem occurred evaluating root project 'myApp'.
> Task with name 'CreateMessageKeys' not found in root project 'mainApp'.

The task definition does not look correct, see defining tasks.

It should look like this:

task ('CreateMessageKeys', type: CreateMessageKeysTask) {
    destDir = "bundle-common/src/";
    outputClass = "common.messages.MessageKeys";
}

or

task (CreateMessageKeys, type: CreateMessageKeysTask) {
    destDir = "bundle-common/src/";
    outputClass = "common.messages.MessageKeys";
}

Irrelevant to this question:

In your second example, perhaps the task CreateMessageKeys does not exist yet when this is evaluated. One possible workaround would be the following:

subprojects.each {
    it.afterEvaluate {
         it.test.dependsOn(...)
    }
}

Or simply putting the task definition above this block could resolve this.