SAP Message WXP504 - Only one quantity key figure can be assigned to interface &1

Diagnosis
You want to create an allocation table usinginterface &V1& for planning step &V3&. Theinterface must contain a quantity key figure before this can be done.Possible key figures are all key figures thatcontain a unit of measure and which are assigned to the planning step.In the interface in question, however, no key figure has been entered.

System Response
No allocation table can be created.

Procedure
Assign a quantity key figure to interface &V1&.