Error in XSOdata Simple Association

Hello Experts ,

Am getting the error while activating a simple association the code is as below :

service {
"sampleschema"."VBAK" as "POHeader";
"sampleschema"."VBAP" as "POItems";
association "Association" with referential constraint principal "POHeader"("VBELN")
multiplicity "1" dependent "POItems"("VBELN") multiplicity "*";
}

Below is the error :

Please help me in this

Thanks,

Aisurya