Sales Orders are inventory records that depend on several backend processes, such as stock reservation, price book validation, tax calculation, product line item checks, inventory adjustments, and linking to related records like Quotes or Invoices. These operations cannot be reliably handled by standard workflow actions.
Because of this, Vtiger restricts creating Sales Orders when the primary module is a non-inventory module like Deals. To create a Sales Order via workflow, the primary module must be an inventory module, such as Invoices.
To learn more about Workflows, click here.