Assertions with XPATH

Sorry if this is in wrong area, but I am new to SOAP UI os.  

I am currently trying to achieve a simple test case where if I call a service with no parameters, the response should not provide me any information back.  Super simple. 

With that, I am trying to get an understanding of xpath assertions. 

I used xpath in selenium tests.  

But I do not understand what I am doing wrong on the Xpath match configuration screen. 

I see that when I click Declare, it provides me namespaces in XPath Expression box. 

It provides this: 

declare namespace soap='http://www.w3.org/2003/05/soap-envelope';
declare namespace ns1='https://ForceDevelopmentServices.ServiceCode.DCPDS.EducationData.DataContracts';
declare namespace ns2='http://schemas.microsoft.com/2003/10/Serialization/';
declare namespace ns3='http://tempuri.org/';
declare namespace ns4='https://USAF.A1.Personnel.ForceDevelopment.DAL.Services.Base';

Under Assertions, 

I used a online xpath tester http://www.freeformatter.com/xpath-tester.html#ad-output to get this xpath expression:

 //for1:SSAN/text()

I would expect to place a ? in the expected results box, but when I click on Select from current I get an error message saying "Missing content to select from".  I then tried to click on the 'Test' to see what happens, I get another error message saying "Missing Content"

I looked over Query/Match, I did not get much from this view since it does not seem to do anything when I put in xpath query or Matching Value

Any assistance would be greatly appreciated.  Thanks in advance.

alex

<soap:Envelope 
xmlns:soap="http://www.w3.org/2003/05/soap-envelope" 
xmlns:for="https://ForceDevelopmentServices.ServiceCode.DCPDS.EducationData.Interfaces" 
xmlns:for1="https://ForceDevelopmentServices.ServiceCode.DCPDS.EducationData.DataContracts">
   <soap:Header/>
   <soap:Body>
      <for:GetAcquisitionsCertificationsBySSANResponse>
         <!--Optional:-->
         <for:GetAcquisitionsCertificationsBySSANResult>
            <!--Zero or more repetitions:-->
            <for1:DCPDSAcquisitionsCertification>
               <!--Optional:-->
               <for1:SSAN>?</for1:SSAN>
               <!--Optional:-->
               <for1:ACQ_DT_CAREER_LVL_ACHVD>?</for1:ACQ_DT_CAREER_LVL_ACHVD>
               <!--Optional:-->
               <for1:ACQ_CAREER_LEVEL_ACHVD_DESC>?</for1:ACQ_CAREER_LEVEL_ACHVD_DESC>
               <!--Optional:-->
               <for1:ACQ_CAREER_LVL_APPR_AUTH>?</for1:ACQ_CAREER_LVL_APPR_AUTH>
               <!--Optional:-->
               <for1:ACQ_CAREER_LVL_APPR_AUTH_DESC>?</for1:ACQ_CAREER_LVL_APPR_AUTH_DESC>
               <!--Optional:-->
               <for1:ACQ_CERT_CAREER_FIELD>?</for1:ACQ_CERT_CAREER_FIELD>
               <!--Optional:-->
               <for1:ACQ_CERT_CAREER_FIELD_DESC>?</for1:ACQ_CERT_CAREER_FIELD_DESC>
            </for1:DCPDSAcquisitionsCertification>
         </for:GetAcquisitionsCertificationsBySSANResult>
      </for:GetAcquisitionsCertificationsBySSANResponse>
   </soap:Body>
</soap:Envelope>