Hi ,
I am able to get API response from Jira cloud API from browser if I have already login in Jira cloud through browser.
However I am not getting response and getiing autherization error 401 if I hit API from PostMan or Java code.
You may want to read https://developer.atlassian.com/cloud/jira/platform/rest/v2/#authentication and check the requirements for setting the proper authentication headers when connecting to the Jira REST API.