Scenario:
- While creating sales order with a markup item as well as a discount item. Error throws “type”:”error.SuiteScriptError”,”name”:”USER_ERROR”,”message”:”Posting and non-posting discounts/markups are not allowed on the same transaction”.
2. When we try to apply promotion in a sales order from webstore/netsuite where we are adding a markup item also through script.
Solution
Consider a salesorder where we are trying to add markup item as well as a promotion.
Check the markup item and from the accounting subtab you can understand whether it is posting or non posting. If there is an account associated with it, then it is a posting markup item.

Similarly check the Promotion or discount item.
In the promotion record, there is option to choose the discount item. If it is partner discount, then it will be non posting(not associated with accounts). If we try to apply such promotion along with the above markup item, then it will throw error.

If the discount item is changed to another one which is posting.

Discount item:

Now both the markup and discount items are posting/accounting. Hence the sales order can be saved without any error. Or another way is to make both non posting, which means not associated with any discounts.
Note:
NetSuite will not allow to use both Posting and non-posting discounts/markups items in the same transaction.
To be able to save the transaction, choose only one type of Discount/Markup items – either posting or non-posting.
This is covered by Enhancement # 156967 – Please have the ability to add a non posting discount and a posting discount on the same transaction.