For example, trying to run the code (I changed the issue key)

def issue = jiraGetIssue site: "JIRA", idOrKey: 'PROJ-12345'
echo issue.data.toString()

gives the error message:
ERROR: {"errorMessages":["You do not have the permission to see the specified issue.","Login Required"],"errors":{}}
but on the plugin configuration page, a user (that has correct permissions in JIRA) is set up with correct username and password – testing connection shows success. 

• Check your JIRA project settings that this user has the permission to view that issue, as this error message is from the JIRA.
• Hope you have saved the config after you see the test connection success message.
• What version are you testing this against, I tested this only with 6.4.13.
• Which type of authentication are you trying to use Basic/OAuth.
• Can you try a simple http get on that issue outside this plugin to see if that is working with credentials you have. (with basic username and password)

• The user is configured to access the issue (and do much more).
• The config was saved – double checked, and the testing connection still shows success
• I am using the plugin (JIRA Steps) version 1.2.2, and JIRA version 7.3.6
• Using basic auth (which works fine in HTTP request)
• HTTP get request works fine outside of the plugin with the same credentials

From looking at the error message, it seems that the problem is that it is not logging into JIRA with the user configured in the plugin settings, but instead using the Anonymous user which does not have permission to view the issue (or do much else). 

Thanks nrayapati

jdudatt I tried to replicate your scenario, and couldn't reproduce the issue that you are reporting. Not sure where you are trying different than me, but please find the screenshots.

Here is the pipeline script:

def issue = jiraGetIssue idOrKey: 'TEST-1', site: 'JIRA'
echo issue.data.fields.description.toString()
jiraAddComment idOrKey: 'TEST-1', site: 'JIRA', comment: 'Testing from Jenkins'

There is no way that this plugin tries as an anonymous, it always uses either Basic or OAuth method, it could be that username and password that you have saved is not allowed to login, or the JIRA version that you have installed is not supporting basic authentication.

Or may be not sure if the Jenkins version is causing some problems, what jenkins version you have installed?

Refer to the signinginterceptor here https://github.com/jenkinsci/jira-steps-plugin/blob/master/src/main/java/org/thoughtslive/jenkins/plugins/jira/login/SigningInterceptor.java#L32-L39 if you would like to try manually posting a request with this header.