SAP Message WTADDI213 - Purchasing document & currently blocked

Diagnosis
You wish to have the system generate or change a purchase order for anadditional automatically. In order for this tobe possible, purchasing document &V1&, towhich the additionals document is assigned, must be blocked. However,this document is currently being processed.
No follow-on purchase order is generated, as processing the purchasingdocument could result in changes being made to the additionalsdocument. For example, if you make changes now to thepurchase order quantity or delivery date
in an item in purchasing document &V1&, this may lead to changes inthe additionals document.

System Response
The purchase order for the additional is not generated/changed.

Procedure
Try again later.