I was trying to download a plugin in Spring Tool Suite using Eclipse marketspace.
Getting below screenshot error.

To resolve the error follow below steps.

Open Spring Tool Suite application folder
Now open Spring Tool Suite4.ini folder
Now add below two commands inside Spring Tool Suite4.ini file

-Djavax.net.ssl.trustStore=cacerts 
-Djavax.net.ssl.trustStorePassword=changeit

Now restart Spring Tool Suiteand try downloading the plugin.