web data Json conversion error

Try these:

let
    // Parameters (can be replaced with dynamic inputs or function arguments)
    APIKey = "YourAPIKey",
    NumOfRows = "12",
    PageNo = "1",
    DataType = "JSON",
    Today = "20241204",           // Example date; replace with dynamic logic if needed
    RecentBaseTime = "0800",      // Example time; replace with dynamic logic if needed
    NX = "61",
    NY = "110",
    
    // Query Parameters as a Record
    QueryParameters = [
        serviceKey = APIKey,
        numOfRows = NumOfRows,
        pageNo = PageNo,
        dataType = DataType,
        base_date = Today,
        base_time = RecentBaseTime,
        nx = NX,
        ny = NY
    ],

    // Build Query String
    QueryString = Uri.BuildQueryString(QueryParameters),

    // API Request
    Source = Json.Document(Web.Contents(
        "https://apis.data.go.kr/1360000/VilageFcstInfoService_2.0/getVilageFcst?",
        [Query = QueryParameters]
    ))
in
    Source

 QueryString = Uri.BuildQueryString(QueryParameters) will return serviceKey=YourAPIKey&numOfRows=12&pageNo=1&dataType=JSON&base_date=20241204&base_time=0800&nx=61&ny=110

let
    // Parameters (can be replaced with dynamic inputs or function arguments)
    APIKey = "YourAPIKey",
    NumOfRows = "12",
    PageNo = "1",
    DataType = "JSON",
    Today = "20241204",           // Example date; replace with dynamic logic if needed
    RecentBaseTime = "0800",      // Example time; replace with dynamic logic if needed
    NX = "61",
    NY = "110",
    
    // API Request
    Source = Json.Document(Web.Contents(
        "https://apis.data.go.kr/1360000/VilageFcstInfoService_2.0/",
        [
            RelativePath = "getVilageFcst?",
            Query = [
                serviceKey = APIKey,
                numOfRows = NumOfRows,
                pageNo = PageNo,
                dataType = DataType,
                base_date = Today,
                base_time = RecentBaseTime,
                nx = NX,
                ny = NY
            ]
        ]
    ))
in
    Source