If you see this, or a similar error, it means that the field is missing from the create or update screen in JIRA. Commonly the error begins with
"Remote error for 'create_feature': Data not accepted: customfield_10600: Field 'customfield_10600' cannot be set. It is not on the appropriate screen, or unknown."
"fixVersions: Field 'fixVersions' cannot be set. It is not on the appropriate screen, or unknown."
Aha! records whether a JIRA field is present when you click the "Load project data" button for the integration, so it is possible that the field has been removed since then or is not on the screen for creating issues.
Note that field configuration may be different for different users, so you should test in JIRA using the same user account that the integration is set up with. Then ensure that the field exists and is on the create and edit screen.
If the error is related to the "Aha! Reference" field and it's no longer in JIRA, you should go to the Aha! integrations screen and reload your project data for JIRA again, this will recreate the field.