Thank you for posting a question on the community! From the error, seems you haven't configured the right maven repositories in your development environment. Other people on the community have experience similar issues previously: https://community.developer.atlassian.com/t/confluence-maven-plugin-error/61186/2. The plugin is not available in maven central, and you need to add the Atlassian repository to your settings.xml, unless you have configured a nexus or someting else that mirrors the Atlassian repositories.
I don't know which os you are running, so keep in mind that paths are os dependent. If you're running Linux or other UNIX flavors, you may likely find the atlassian sdk settings.xml in "/usr/share/atlassian-plugin-sdk-8.2.8/apache-maven-3.5.4/conf/settings.xml". On Windows this would probably be "C:\Applications\Atlassian\atlassian-plugin-sdk-8.2.7\apache-maven-3.5.4\conf\settings.xml", and for Mac i'm not sure where this is installed. The needed repository is probably https://maven.atlassian.com/repository/public. If you add the repository found in the above settings.xml file to your local .m2/settings.xml, and also to the <repositories> tag in your pom.xml, you should be able to install the needed package.