Can you check that this repository is configured:
https://repository-plugtree-org.forge.cloudbees.com/snapshot/
If so, you should register that repository in your CACERTS certificates, for doing that you can follow this steps:
https://cloudbees.zendesk.com/entries/20078393-configuring-maven-to-access-https-repositories-with-untrusted-ssl-certificates
Doing this you will trust in this repository to download dependencies.
Cheers