Summary

Try these steps to identify the root cause if the users will get the below error message while creating any issue in Jira Cloud.

Error message "User 'User' is not valid for this user picker"

This problem is usually reported when any "User picked" custom field associated with that screen is having an invalid username set as the default value. 

Environment

Cloud

Diagnosis and Solution

We need to first identify the custom fields that are 

• Open browser dev-tool.
• Try to replicate the issue and look out for any HTTP 400 error, as below.

• Request URL:
  https://<Cloud Site>.atlassian.net/rest/api/2/issue
  Request Method:
  POST
  Status Code:
  400

• Identify the custom field details from the response tab from this endpoint call. You will see a similar message as below.

• {"errorMessages":[],"errors":{"customfield_15240":"User 'User ' is not valid for this user picker."

  Now that we have identified the custom field ID, navigate to the Jira Global admin → Custom field page. Go to the customfield context and delete the default value, as below