Plugin with the Extractor2 interface fails to install

Hello Atlassian-Community,

I’m having trouble installing my plugin using the Extractor2 interface. The frontend told me to have a look at the log, but I’m not able to find out what causes the problem. Can anyone help me?

Log

2019-08-12 14:21:07,002 ERROR [UpmAsynchronousTaskManager:thread-2] [upm.core.install.ObrPluginInstallHandler] logUnsatisfiedRequirements OBR resolver has 1 unsatisfied requirements for installing plugin_3747237097166841438_WikiDoc-1.0.0-SNAPSHOT.obr
 -- url: /rest/plugins/1.0/ | referer: http://localhost:8090/plugins/servlet/upm | traceId: 6f5753941b2d3432 | userName: admin
2019-08-12 14:21:07,002 ERROR [UpmAsynchronousTaskManager:thread-2] [upm.core.install.ObrPluginInstallHandler] logUnsatisfiedRequirements Unsatisfied requirement: (&(package=com.atlassian.confluence.index.api))
 -- url: /rest/plugins/1.0/ | referer: http://localhost:8090/plugins/servlet/upm | traceId: 6f5753941b2d3432 | userName: admin
2019-08-12 14:21:07,044 WARN [UpmAsynchronousTaskManager:thread-2] [upm.core.install.DefaultPluginInstallationService] execute Plugin installation failed: Failed to resolve plugin dependencies within OBR [plugin_3747237097166841438_WikiDoc-1.0.0-SNAPSHOT.obr]. Please see the logs for more detailed information.
 -- url: /rest/plugins/1.0/ | referer: http://localhost:8090/plugins/servlet/upm | traceId: 6f5753941b2d3432 | userName: admin

atlassian-plugin.xml

	<extractor2 name="Description extractor"
				key="descriptionextractor"
				class="com.example.demo.impl.DescriptionPropertyExtractor">
		<description>Demo</description>
	</extractor2>

Confluence Server 6.14.7
ATLAS 8.0.16