No JavaDoc is released for artifact org.apache.nifi:nifi-standard-services-api-nar:1.21.0. Please try other versions.

Owner of the project? Troubleshoot following things:

• Retry this version. Sometimes it's just a matter of time to pick up new versions.
• Is the nifi-standard-services-api-nar-1.21.0-javadoc.jar actually distributed to either maven or bintray?
• Does the nifi-standard-services-api-nar-1.21.0-javadoc.jar file contains an index.html in the extracted root folder? (you can rename .jar to .zip to extract the file)

report to us if you tried everything but still doesn't work.