The [Releasing a project?|https://jenkinsci.github.io/jira-steps-plugin/getting-started/examples/release_project/] and [the project homepage|https://jenkinsci.github.io/jira-steps-plugin/] code examples contains the following lines:

def transitionInput = [transition: [name: 'Close']]
jiraTransitionIssue idOrKey: issueKey, input: transitionInput

def transitionInput = [ transition: [ name: 'Close'] ]
jiraTransitionIssue idOrKey: 'RELEASE-999', input: transitionInput, site: 'YOURJIRASITE'

However, this is not valid for the Jira API endpoint: https://developer.atlassian.com/cloud/jira/platform/rest/v2/#api-rest-api-2-issue-issueIdOrKey-transitions-post. The IssueTransition parameter only takes the ID of the issue transition. The documentation needs to be corrected so the function call works.