Jira Cloud has updated the way they handle the users to be compliant to GDPR (Changes to usernames in Jira Cloud | Jira Software Cloud | Atlassian Documentation)
The current Jira plugin action Assign Issue is expecting the Username of assignee as the input string. However, the Jira Cloud doesn’t accept the username anymore, it rather needs the UserID and the API need to be updated. (https://developer.atlassian.com/cloud/jira/platform/deprecation-notice-user-privacy-api-migration-guide/). Currently, the Assign Issue action fails in jira Cloud.