I'm trying to compile multiverse-core from source, with mvn install, and I get this:

[INFO] -------------------------------------------------------
[INFO] T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.onarandombox.multiverseinventories.TestWSharableAPI
[WARNING] Tests run: 1, Failures: 0, Errors: 0, Skipped: 1, Time elapsed: 0.011 s - in com.onarandombox.multiverseinventories.TestWSharableAPI
[INFO] Running com.onarandombox.multiverseinventories.TestPlayerNameChange
[ERROR] Tests run: 3, Failures: 0, Errors: 2, Skipped: 0, Time elapsed: 0.009 s <<< FAILURE! - in null
[ERROR] Test mechanism Time elapsed: 0.001 s <<< ERROR!
java.lang.reflect.InaccessibleObjectException: Unable to make protected native java.lang.Object java.lang.Object.clone() throws java.lang.CloneNotSupportedException accessible: module java.base does not "opens java.lang" to unnamed module @4434095f

[ERROR] Failure when constructing test Time elapsed: 0.001 s <<< ERROR!
java.lang.RuntimeException: PowerMock internal error: Should never throw exception at this level
Caused by: java.lang.reflect.InaccessibleObjectException: Unable to make protected native java.lang.Object java.lang.Object.clone() throws java.lang.CloneNotSupportedException accessible: module java.base does not "opens java.lang" to unnamed module @5ec3689b

[INFO]
[INFO] Results:
[INFO]
[ERROR] Errors:
[ERROR] Unable to make protected native java.lang.Object java.lang.Object.clone() throws java.lang.CloneNotSupportedException accessible: module java.base does not "opens java.lang" to unnamed module @4434095f
[ERROR] PowerMock internal error: Should never throw exception at this level
[INFO]
[ERROR] Tests run: 4, Failures: 0, Errors: 2, Skipped: 1
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 12.168 s
[INFO] Finished at: 2021-06-20T18:01:21Z
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:3.0.0-M3:test (default-test) on project Multiverse-Inventories: There are test failures.
[ERROR]
[ERROR] Please refer to /home/ubuntu/mc-spigot/plugins-source/Multiverse-Inventories/target/surefire-reports for the individual test results.
[ERROR] Please refer to dump files (if any exist) [date].dump, [date]-jvmRun[N].dump and [date].dumpstream.
[ERROR] There was an error in the forked process
[ERROR] Test mechanism :: Unable to make protected native java.lang.Object java.lang.Object.clone() throws java.lang.CloneNotSupportedException accessible: module java.base does not "opens java.lang" to unnamed module @4434095f
[ERROR] org.apache.maven.surefire.booter.SurefireBooterForkException: There was an error in the forked process
[ERROR] Test mechanism :: Unable to make protected native java.lang.Object java.lang.Object.clone() throws java.lang.CloneNotSupportedException accessible: module java.base does not "opens java.lang" to unnamed module @4434095f
[ERROR] at org.apache.maven.plugin.surefire.booterclient.ForkStarter.fork(ForkStarter.java:657)
[ERROR] at org.apache.maven.plugin.surefire.booterclient.ForkStarter.run(ForkStarter.java:283)
[ERROR] at org.apache.maven.plugin.surefire.booterclient.ForkStarter.run(ForkStarter.java:246)
[ERROR] at org.apache.maven.plugin.surefire.AbstractSurefireMojo.executeProvider(AbstractSurefireMojo.java:1161)
[ERROR] at org.apache.maven.plugin.surefire.AbstractSurefireMojo.executeAfterPreconditionsChecked(AbstractSurefireMojo.java:1002)
[ERROR] at org.apache.maven.plugin.surefire.AbstractSurefireMojo.execute(AbstractSurefireMojo.java:848)
[ERROR] at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
[ERROR] at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:210)
[ERROR] at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:156)
[ERROR] at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:148)
[ERROR] at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117)
[ERROR] at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81)
[ERROR] at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:56)
[ERROR] at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
[ERROR] at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:305)
[ERROR] at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:192)
[ERROR] at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:105)
[ERROR] at org.apache.maven.cli.MavenCli.execute(MavenCli.java:957)
[ERROR] at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:289)
[ERROR] at org.apache.maven.cli.MavenCli.main(MavenCli.java:193)
[ERROR] at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[ERROR] at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:78)
[ERROR] at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[ERROR] at java.base/java.lang.reflect.Method.invoke(Method.java:567)
[ERROR] at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:282)
[ERROR] at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:225)
[ERROR] at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:406)
[ERROR] at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:347)
[ERROR]
[ERROR] -> [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/MojoExecutionException

How do I fix this?

Apache Maven 3.8.1 (05c21c65bdfed0f71a2f2ada8b84da59348c4c5d)
Maven home: /opt/apache-maven-3.8.1
Java version: 16.0.1, vendor: Private Build, runtime: /usr/lib/jvm/java-16-openjdk-amd64
Default locale: en, platform encoding: UTF-8
OS name: "linux", version: "5.8.0-1035-aws", arch: "amd64", family: "unix"