How to take input of a nested JSON in FastAPI?

Question

{ "publicationSystem": "xyz",
       "deliveryMethod": "Queued",
       "recipients": [
      {
         "identifiers": [
            {
               "type": "CustomerId",
               "value": "123456783"
            }
         ]
      }
   ],
   "templateId": "123453",
   "templateFields": [
      {
         "id": "fuhu",
         "value": "Jojo"
      }
   ]
}

I'm using pydantic but still whenever i'm passing the json i get the error
Error: Unprocessable Entity

def extract_values(dct, lst=[], keys=[]):
    jumbo={}
    if not isinstance(dct, (list, dict)):
        lst.append(('_'.join(keys), dct))
    elif isinstance(dct, list):
        for i in dct:
            extract_values(i, lst, keys)
    elif isinstance(dct, dict):
        for k, v in dct.items():
            # print(k)

            keys.append(k)
            extract_values(v, lst, keys)
            keys.remove(k)
            # print(lst)
    for i in lst:
        jst = []
        i = list(i)
        jst = i[0].split("_")
        jumbo[jst[-1]] = i[1]
    return jumbo

this is the function i'm using to get the key value pairs of my json file but its showing error while taking input of the nested json string