How To Fix V1821 - Do not mix schedule lines with and without purchase order types

SAP Error Message - Details

• Message type: E = Error

• Message class: V1 - Verkaufsbelegbearbeitung

• Message number: 821

• Message text: Do not mix schedule lines with and without purchase order types

What causes this issue?

You have tried to create a schedule line for an item which already has
other schedule lines. These exisiting schedule lines refer to order
type '&V3&'.
The schedule line you just entered has schedule line category '&V1&'.
Order type '&V2&' belongs to this category.
The error message occurred because either order type '&V2&' or '&V3&'
is empty.

How to fix this error?

Change the schedule line categories so that either all or none of the
schedule lines for the item have an order type.

Error message extract from SAP system. Copyright SAP SE.