Cvc-elt.1: Cannot Find The Declaration Of Element 'soap:Envelope'

First, you must add a targetNamespace="http://tempuri.org/" to the xs:schema element of your XSD in order for your XSD to apply to the namespace used in your XML payload.

Then you can take either of the following approaches: Change the XML or Change the XSD. Choose according to which files you control.

Change the XML

Add

xsi:schemaLocation="http://tempuri.org/ tempuri.xsd
                    http://schemas.xmlsoap.org/soap/envelope/ 
                    http://schemas.xmlsoap.org/soap/envelope/

to the soap:Envelope:

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
               xmlns:xsd="http://www.w3.org/2001/XMLSchema"
               xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
               xsi:schemaLocation="http://tempuri.org/ tempuri.xsd
                                   http://schemas.xmlsoap.org/soap/envelope/ 
                                   http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <Cancel_OrderLine xmlns="http://tempuri.org/">
      <Data>
        <Delivery>
          <Delivery_No>1605000194</Delivery_No>
          <Reason>qwertyu</Reason>
        </Delivery>
        <Delivery>
          <Delivery_No>1605000194</Delivery_No>
          <Reason>qwerty</Reason>
        </Delivery>
      </Data>
    </Cancel_OrderLine>
  </soap:Body>
</soap:Envelope>

Change the XSD

Add

<xs:import namespace="http://schemas.xmlsoap.org/soap/envelope/"   
           schemaLocation="http://schemas.xmlsoap.org/soap/envelope/"/>

to the xs:schema element of your XSD:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
           targetNamespace="http://tempuri.org/">

  <xs:import namespace="http://schemas.xmlsoap.org/soap/envelope/"   
             schemaLocation="http://schemas.xmlsoap.org/soap/envelope/"/>

  <xs:element name="Cancel_OrderLineReq">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="Data">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Delivery" maxOccurs="unbounded" minOccurs="0">
                <xs:complexType>
                  <xs:sequence>
                    <xs:element type="xs:int" name="Delivery_No"/>
                    <xs:element type="xs:string" name="Reason"/>
                  </xs:sequence>
                </xs:complexType>
              </xs:element>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

Both methods will allow successful validation of your SOAP envelop and its payload.