Shippable "Failed to execute goal org.codehaus.mojo:cobertura-maven-plugin:2.7:instrument"

Question

I am currently doing a software engineering module where we learn about CI and TDD. As an assignment we must create a Java project using maven, JUnit, and Shippable. My files include three java classes (Student, Course, and Module classes) and a simple test for each that tests getters and setters.

I am using:

1. Apache NetBeans 12.0
2. Maven (default is 3.6.3 but I added a plugin for 3.8)
3. Openjdk11

My pom file:

<?xml version="1.0" encoding="UTF-8"?>
<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>com.nuig</groupId>
    <artifactId>assignment1</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
    </properties>
    <dependencies>
        <!-- https://mvnrepository.com/artifact/joda-time/joda-time -->
        <dependency>
            <groupId>joda-time</groupId>
            <artifactId>joda-time</artifactId>
            <version>2.10.6</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/org.junit.jupiter/junit-jupiter-api -->
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-api</artifactId>
            <version>5.7.0</version>
            <scope>test</scope>
        </dependency>    
    </dependencies>
    <name>assignment1</name>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.0</version>
                <configuration>
                    <release>11</release>  <!--or <release>10</release>-->
                </configuration>
            </plugin>
    </plugins>
    </build>
</project>

And shippable.yml file:

language: java
jdk:
- openjdk11
after_success:
- mvn clean verify 
- mvn clean cobertura:cobertura
- mvn test

When I build the code in netbeans, it builds and passes the tests fine. But once I push and go to shippable console, it keeps failing at mvn clean cobertura:cobertura with the following error:

[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  17.165 s
[INFO] Finished at: 2020-10-11T00:26:35Z
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.codehaus.mojo:cobertura-maven-plugin:2.7:instrument (default-cli) on project assignment1: Execution default-cli of goal org.codehaus.mojo:cobertura-maven-plugin:2.7:instrument failed: Plugin org.codehaus.mojo:cobertura-maven-plugin:2.7 or one of its dependencies could not be resolved: Could not find artifact com.sun:tools:jar:0 at specified path /usr/lib/jvm/java-11-openjdk-amd64/../lib/tools.jar -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/PluginResolutionException

I am really unfamiliar with all these technologies and I have already tried several suggestions from answers to similar questions but to no vail.

Would anyone be able to help with this issue? Much appreciated.