consuming xml webservice using retrofit Parameter `soap12:Body` does not have a match in class

You are missing one more class to to hold Body for request.

That is Envelope will contain Body object and Body will contain GetCountriesAvailable object.

Example:

@Root(name = "Envelope")
@NamespaceList(value = [
    Namespace(prefix = "xsi", reference = "http://www.w3.org/2001/XMLSchema-instance"),
    Namespace(prefix = "xsd", reference = "http://www.w3.org/2001/XMLSchema"),
    Namespace(prefix = "soap12", reference = "http://www.w3.org/2003/05/soap-envelope")
])
data class CountriesAvailableRequestEnvelope(
    @Element(name = "Body", required = false)
    @Namespace(reference="http://www.w3.org/2003/05/soap-envelope", prefix:"soap12")
    val Body: Body)


@Root(name = "Body", strict = false)
@Namespace(reference="http://www.w3.org/2003/05/soap-envelope", prefix:"soap12")
data class Body(
    @Element(name = "GetCountriesAvailable", required = false)
    val countriesAvailableRequest: CountriesAvailableRequest)



@Root(name = "GetCountriesAvailable", strict = false)
@Namespace(reference = "http://www.holidaywebservice.com")
data class CountriesAvailableRequest(
    @Element(name = "GetCountriesAvailable", required = false) val code: String)

Do not include prefix as part of element name.

Change

@Root(name = "soap12:Envelope")
@NamespaceList(value = [
    Namespace(prefix = "xsi", reference = "http://www.w3.org/2001/XMLSchema-instance"),
    Namespace(prefix = "xsd", reference = "http://www.w3.org/2001/XMLSchema"),
    Namespace(prefix = "soap12", reference = "http://www.w3.org/2003/05/soap-envelope")
])

to

@Root(name = "Envelope")
@NamespaceList(value = [
    Namespace(prefix = "xsi", reference = "http://www.w3.org/2001/XMLSchema-instance"),
    Namespace(prefix = "xsd", reference = "http://www.w3.org/2001/XMLSchema"),
    Namespace(prefix = "soap12", reference = "http://www.w3.org/2003/05/soap-envelope")
])

---

And for Body element change as below.

@Root(name = "Body", strict = false)
 @Namespace(reference="http://www.w3.org/2003/05/soap-envelope")
data class CountryCode(
    @Element(name = "description", required = false) val description: String?,
    @Element(name = "code", required = false) val code: String?)