How to set the Status of Return Authorization to Pending Receipt via Workflow?

Hi There

I am creating a workflow to route a return authorisation transaction through multiple individuals.

I am getting stuck with not being able to set the native 'order status' field via the workflow on approval.

The workflow has removed the native 'approve return' button and replaced it with a custom button.

I am trying to set the native field, 'Order Status': (field id: orderstatus) when the user clicks the custom 'approve' button.

However, the list of options in the workflow for that field do not match the available statuses on the record:

There is no pending receipt in here per the above screenshot.