SAP Message AO287 - Alternate approp. request &1 is being processed by user &2

Diagnosis
Appropriation request &V1& is blocked by user &V2&. Appropriationrequest &V1& is one of the alternate requests to appropriation requestV3 and cannot be processed at this time.

System Response
The system does not allow parallel processing of requests.

Procedure
Wait until user &V2& has finished processing the appropriation request.