This article lists and describes common errors that my occur with the Jira integrations (Epics or Projects)
The OnePlan plan does not have a value in the Jira Project Type field populated.
Jira Epics or Jira Projects
"assignee":"Field 'assignee' cannot be set. It is not on the appropriate screen, or unknown."
In Jira, in order to update a field with data, the field must be on the Issue form. The field that is referenced in the error, is not on the issue form. Based on the rules in Jira, it cannot be updated with any data. The solution is to add the field to the form.