minoseah629,
Namespaces can be confusing, once you get how they work, then it should be easy.
Regarding Namespaces:
#1 Elements GetAcquisitionsCertificationsBySSANResponse, GetAcquisitionsCertificationsBySSANResult are belong to namespace "https://ForceDevelopmentServices.ServiceCode.DCPDS.EducationData.Interfaces"
#2 And rest of the elements are belong to namespace "https://ForceDevelopmentServices.ServiceCode.DCPDS.EducationData.DataContracts"
Note that one needs to be aware of namespaces.
If you see the actual response, elements of #1 have prefix "for", but in the xpath below, the same namespace is mapped to prefix "ns1".
Similarly, elements of #2 have prefix "for1", but in the xpath, the same namespace is mapped to prefix "ns2".
In the XPath assertion, you have to use the same prefixes that you declare ( and should not concern about prefix that is actually present in the response.)
XPath Assertion:
declare namespace ns1='https://ForceDevelopmentServices.ServiceCode.DCPDS.EducationData.Interfaces';
declare namespace ns2='https://ForceDevelopmentServices.ServiceCode.DCPDS.EducationData.DataContracts';
//ns1:GetAcquisitionsCertificationsBySSANResponse/ns1:GetAcquisitionsCertificationsBySSANResult/ns2:DCPDSAcquisitionsCertification/ns2:SSAN/text()
Also there is another way for the same xpath, to use any namespace using "*"
//*:GetAcquisitionsCertificationsBySSANResponse/*:GetAcquisitionsCertificationsBySSANResult/*:DCPDSAcquisitionsCertification/*:SSAN/text()
In the above full xpath is mentioned. You can also use the short xpath, like you tried, only if that element is unique in the response.
declare namespace ns2='https://ForceDevelopmentServices.ServiceCode.DCPDS.EducationData.DataContracts';
//ns2:SSAN