INACTIVE_OWNER_OR_USER: operation performed with inactive User

By default, Salesforce refuses to let you set a record's owner (or any user lookup) to a deactivated user. The reasoning is straightforward: the inactive user can't act on the record, can't be notified, can't approve workflows.

What triggers it

Account a = new Account(Name = 'Test', OwnerId = formerEmployeeId);
insert a;     // INACTIVE_OWNER_OR_USER

Or:

Case c = [SELECT Id FROM Case WHERE Id = :caseId];
c.OwnerId = formerEmployeeId;
update c;     // INACTIVE_OWNER_OR_USER

Fix 1: reassign to a queue or active user

Most-active-user fix: query the user's role/team, find an active substitute, assign there.

List<User> backups = [
    SELECT Id FROM User
    WHERE Profile.Name = 'Sales User'
      AND IsActive = true
      AND UserRoleId = :originalUser.UserRoleId
    LIMIT 1
];
if (!backups.isEmpty()) {
    a.OwnerId = backups[0].Id;
}

For systematic re-ownership during deactivation, use a mass-transfer workflow: when a user is being deactivated, run a script to transfer their records to their manager (or a queue) before flipping the IsActive bit.

Fix 2: reactivate the user

If the deactivation was accidental:

Setup → Users → click the user → uncheck Active to reactivate.

Note: reactivating uses a license seat. If your org is at its license cap, you can't reactivate without freeing a seat elsewhere.

Fix 3: enable the org setting

Setup → Customize → Activities → Activity Settings (and similar pages for other features) sometimes have a checkbox: "Allow inactive User to be assigned as Owner of [object]." Toggle on, and the platform stops throwing this error for that specific object's ownership.

But: most teams want this validation. Disabling it lets ownership rot over time as users come and go.

Fix 4: disable the validation entirely

Org-wide, you can disable the check via Setup → User Management Settings → "Allow Inactive Users to Reference" (the exact name varies by Salesforce version). This is org-wide and applies to all objects.

Don't do this. The validation is one of the cheap defences against ownership rot.

A subtle case: lookup fields that aren't Owner

The error fires for any User lookup, not just OwnerId. If you have a custom Account_Manager__c lookup to User and try to set it to a deactivated user, you get this error. The same fixes apply.

Different from "user can't see the record"

Don't confuse this with INSUFFICIENT_ACCESS_OR_READONLY. That fires when an active user lacks access. INACTIVE_OWNER_OR_USER fires specifically because the user is deactivated, regardless of their permissions when they were active.