Hi, there are a few things you can try:

1- Security Constraints: Check if there are any specific security rules applied to the model that might prevent the copy() method from being executed. You may need to tweak access control or create a custom group to allow this operation.

2- Permission Issues: Make sure that the user or group running the automated action has the proper permissions to access and modify work orders. Sometimes these errors are related to insufficient access rights.

3- Alternative Solution with Scheduled Action:
If the execute python code method is not working, you could consider using a scheduled action to periodically check for completed dummy work orders and duplicate them with a new number.

Here’s an example of how you might implement this:
- Create a scheduled action that runs periodically (like every minute or hour).
- Check for work orders that are in the "done" state.
- For each completed work order, copy it, reset the fields, and assign a new sequence number.

Python code for the scheduled action:

workorders = self.env['mrp.workorder'].search([('state', '=', 'done'), ('production_id', '!=', False)])
for wo in workorders:
new_wo = wo.copy({
'state': 'ready',
'time_ids': [(5, 0, 0)],
'qty_produced': 0,
})
new_wo.name = self.env['ir.sequence'].next_by_code('mrp.workorder') or new_wo.name

This way, the scheduled action runs automatically without needing the "execute Python code" method within the automated action.

4- Override the Forbidden Opcode Error (if applicable):
If you believe the code should run as intended, you could try overriding the security rules for the specific field or action causing the issue. However, this could have security implications, so it's better to first test with less risky solutions like the scheduled action approach.

Let me know if any of these suggestions help.