1. Multiple Shipping Routes is enabled under Setup > Company > Enable Features > Transactions tab > Shipping & Receiving section.
2. When a user enters or edits a sales order with Enable Item Line Shipping = T, below error message is displayed as soon as a Promotion or Coupon Code is selected:
"When the transaction has Multiple Shipping Route on, promotion with shipping impact is not allowed, and will be removed".
User must review the Promotion Code setup under Lists > Marketing > Promotions. The warning or prompt message is displayed only when the Promotion or Coupon Code used has the following setup:
1. Free Shipping Method is selected (not blank); or
2. Display line discounts is enabled
This is currently an enhancement request logged under Enhancement 267108: the capability to use advanced promotions for Multiple Shipping Routes.