from US-Core-R4/source/pages/general-guidance.md:

{ "resourceType" : "Patient", ... "name":[ "extension" : [ "url" : "http://hl7.org/fhir/StructureDefinition/data-absent-reason", "valueCode" : "unknown" }

]
]
"telecom" :
...
}
The actutal data absent reason extension above is invalid JSON. it should be:

{
"resourceType" : "Patient",
...
"name":[{
"extension" : [

{ "url" : "http://hl7.org/fhir/StructureDefinition/data-absent-reason", "valueCode" : "unknown" }

]
}]
"telecom" :
...
}