Unable to find the mistake on the payload that is returning 400 response

Question

I have the following script that are returning response 400 but I am unable to find where the mistake in the payload is.

I am getting an error that raise this: JSONDecodeError("Expecting value", s, err.value) from None I assume that the payload is with wrong data format, but I have running this without the loop and it has been working.

API_Key = os.getenv("API_Key")

url = "https://app.pendo.io/api/v1/aggregation"

dates = pd.date_range(start='1/1/2022', end='10/1/2022',
                      freq='M').strftime("%Y-%m-%d").tolist()

list_epoch=[]

for i in dates:
    epoch = int(time.mktime(time.strptime(i, "%Y-%m-%d")))
    list_epoch.append(epoch)


days = [31,28,31,30,31,30,31,31,30,31,30,31,31,28,31,30,31,30,31,31,30]


for var1, var2 in zip(list_epoch,days):
    data_payload = {
        "response": {
            
            "mimeType": "application/json"
        },
        "request": {
            
            "pipeline": [
                {
                    "source": {
                        "events":{
                            "featureId": "iNdfepY576Yo-x1itQUp5T99z8Y"
                        },
                        
                        "timeSeries": {
                            "first": var1,
                            "count": -var2,
                            "period": "dayRange"                      

                        }
                    }
                }           
            ]
        }
    }
    print(data_payload)
    headers = {
        'x-pendo-integration-key': API_Key,
        'content-type': "application/json"
    }
    
    response = requests.post(url, json=data_payload, headers=headers)
    print(response)
    response_dictionary = json.loads(response.content)

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