I think you're in a company and that Internet access is through a internet proxy.
This proxy is a Main in the Middle proxy : Java Launch JMeter https client from Plugins manager == (https connection 1) => Internet proxy with X509 certificats from your company == (http connection 2) => web site
https://jmeter-plugins.org.
You need to add the certificate chain (X509 Certificats from your company) to the cacerts file of the JDK that launches JMeter.
I have to do these actions when I am on the network of the company where I work.
Regards.
Vincent DAB.