Gradle build issue with 6.9.1

[manmedia]

Hello,

After checking out the project and calling gradle build I am seeing the following error

* What went wrong:
A problem occurred evaluating project ':simpleslackapi-lib'.
> Could not get unknown property 'sourcesJar' for object of type org.gradle.api.publish.maven.internal.publication.DefaultMavenPublication.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.9.1/userguide/command_line_interface.html#sec:command_line_warnings

I thought the build should just pass, because it's the release code?