Out of the box, Maven 3 only supports file:, http: and https: as transport protocols. Maven 2 adds scp: to this list.
If you try to deploy a site with an unsupported protocol, you'll get an error:
[ERROR] Unsupported protocol: '...' for site deployment to distributionManagement.site.url=... Currently supported protocols are: ... Protocols may be added through wagon providers. For more information, see http://maven.apache.org/plugins/maven-site-plugin/examples/adding-deploy-protocol.html
As explained in the error message, to deploy a site with a non-built-in protocol, you need to add the corresponding wagon provider.
You can add it either as a global extension, or declare it at as a dependency of the site plugin:
<project> ... <build> <pluginManagement> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-site-plugin</artifactId> <version>3.9.0</version> <dependencies> <dependency><!-- add support for ssh/scp --> <groupId>org.apache.maven.wagon</groupId> <artifactId>wagon-ssh</artifactId> <version>1.0</version> </dependency> </dependencies> </plugin> </plugins> </pluginManagement> </build> ... </project>