Reservation &1 is already being processed by &2

What causes this issue?

Reservation &V1& is being processed by user &V2&.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

Wait until user &V2& has finished processing the reservation, or switch
to display mode.

Error message extract from SAP system. Copyright SAP SE.