Can't run a Scala object in a Maven project in STS (Eclipse)

If I create a Scala project in Spring Tool Suit with a scala object called HelloWorld.scala with just the following lines:

package ScalaMavenTest

object HelloWorld {
 def main(args: Array[String]) {
    println("Hello world !!!")
  } 
}

And then I try to run it, everything goes fine and print out "Hello World !!!"

However, if I convert it to maven STS can't find the main class anymore. Here are the steps I followed:

1-I Converted the Scala Project into Maven:

Right click on the project -> Configure -> Convert to Maven Project -> Finish

2- I changed the standard pom.xml by:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>ScalaMavenTest</groupId>
    <artifactId>ScalaMavenTest</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <repositories>
        <repository>
            <id>scala-tools.org</id>
            <name>Scala-tools Maven2 Repository</name>
            <url>https://oss.sonatype.org/content/groups/scala-tools/</url>
        </repository>
    </repositories>

    <pluginRepositories>
        <pluginRepository>
            <id>scala-tools.org</id>
            <name>Scala-tools Maven2 Repository</name>
            <url>https://oss.sonatype.org/content/groups/scala-tools/</url>
        </pluginRepository>
    </pluginRepositories>

    <dependencies>
        <dependency>
            <groupId>org.scala-lang</groupId>
            <artifactId>scala-library</artifactId>
            <version>2.11.4</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <sourceDirectory>src</sourceDirectory>
        <pluginManagement>
            <plugins>
                <plugin>
                    <groupId>net.alchim31.maven</groupId>
                    <artifactId>maven-scala-plugin</artifactId>
                    <version>3.2.0</version>
                </plugin>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-compiler-plugin</artifactId>
                    <version>3.1</version>
                </plugin>
                <plugin>
                    <groupId>org.scala-tools</groupId>
                    <artifactId>maven-scala-plugin</artifactId>
                    <executions>
                        <execution>
                            <id>scala-compile-first</id>
                            <phase>process-resources</phase>
                            <goals>
                                <goal>add-source</goal>
                                <goal>compile</goal>
                            </goals>
                        </execution>
                    </executions>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>
</project>

3 - Right Click on Project -> Run as Maven Build... -> goals: clean install

4 - I try to run the Scala Object as Scala Application:

4.1 Right Click on Project -> Properties -> Scala Compiler -> select Use Project Settings -> Scala Installation -> Fixed Scala Installation 2.11.8 (built-in)

4.2 Right click on Scala Object -> Run As -> Run Configurations -> Double Click on Scala Application:

     Project: ScalaMavenTest

     Main Class: ScalaMavenTest.HelloWorld

However the output is the following error:

Error: Could not find or load main class ScalaMavenTest.HelloWorld

This questions was already well asked two years ago here:

Running a scala application in maven project

But I tried all the given answers and I still have the same error. I guess the possible solutions need an update or I am missing something. Any extra recommendation?

Extra Details:

Spring Tool Suit: sts-3.8.2.RELEASE

Eclipse Marketplace -> Installed: -Maven (Java EE) Integration for Eclipse WTP (Luna/Mars) 1.2.3 -Scala IDE 4.2.x