Salesforce API: INACTIVE_OWNER_OR_USER Error When Assigning a Task to an Inactive User

Root Cause: Create Audit Fields Permission

The "Create Audit Fields" permission (also known as the "Allow setting of audit fields such as Created By, Created Date, Last Modified By, Last Modified Date via API" permission) allows the API to set system audit fields on records. When this permission is enabled at the org level, Salesforce relaxes enforcement of the INACTIVE_OWNER_OR_USER validation, allowing assignment to inactive users via API. When the permission is disabled (the default state for most orgs), Salesforce enforces the validation and returns the INACTIVE_OWNER_OR_USER error.

How to Check and Enable the Permission

To view or enable the Create Audit Fields permission, contact Salesforce Support to request it be enabled, or refer to the Salesforce Help article linked in Additional Resources for the enablement process.
Note: Assigning records to inactive users via the Salesforce API is only supported through API calls — this cannot be done through the Salesforce user interface.