Need help on groovy script

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Hi,

 I am trying to avoid users from creating a SO when selecting a specific Order Type. I tried the below groovy script but facing errors. Can anyone please help 

 

import oracle.apps.scm.doo.common.extensions.ValidationException;
import oracle.apps.scm.doo.common.extensions.Message;
String CustomerOrderType = header.getAttribute("TransactionTypeCode");
if( customerOrderType == null ) return;
if(CustomerOrderType=="SVC ADV. EXCHANGE")  {
    throw new ValidationException(new Message(Message.MessageType.WARNING, " !!!"));}
return;