request-
<soapenv:Body>
<ns:TravelItineraryModifyInfoRQ Version="2.1.1">
<ns:CustomerInfo>
<ns:CustLoyalty MembershipID="520000001221" NameNumber="02.01"
ProgramID="EY" RPH="1" />
</ns:CustomerInfo>
</ns:TravelItineraryModifyInfoRQ>
</soapenv:Body>
response-
<soap-env:Body>
<TravelItineraryModifyInfoRS Version="2.1.1"
xmlns="http://webservices.sabre.com/sabreXML/2011/10"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:stl="http://services.sabre.com/STL/v01">
<stl:ApplicationResults status="NotProcessed">
<stl:Error type="BusinessLogic" timeStamp="2018-04-20T13:13:39-
05:00">
<stl:SystemSpecificResults>
<stl:Message>.EY/EY FQTV DATA EXISTS FOR THIS PASSENGER.SEE
ITEM 02</stl:Message>
<stl:Message>.ENTRY NOT PROCESSED BEGINNING WITH</stl:Message>
<stl:Message>FF1¤EY100014486283-02.01</stl:Message>
<stl:ShortText>ERR.SWS.HOST.ERROR_IN_RESPONSE</stl:ShortText>
</stl:SystemSpecificResults>
</stl:Error>
</stl:ApplicationResults>
</TravelItineraryModifyInfoRS>
</soap-env:Body>
Is modification not allowed on membership id. http://webservices.sabre.com/drc/servicedoc/TravelItineraryAddInfoLLSRQ_v2.2.0_Sample_Payloads.xml Seems it can be.
It would appear that the FF data for passenger 02.01 is stored in RPH2, and you're attempting to modify 1, while the passenger's data is still in RPH2.
Try modifying the request to RPH 2:
<soapenv:Body>
<ns:TravelItineraryModifyInfoRQ Version="2.1.1">
<ns:CustomerInfo>
<ns:CustLoyalty MembershipID="520000001221" NameNumber="02.01"
ProgramID="EY" RPH="2" />
</ns:CustomerInfo>
</ns:TravelItineraryModifyInfoRQ>
</soapenv:Body>