Heroku: ERROR in app: Exception on /cities/Republi

Question

When user inserts a country name, I am using d3 to clean it and call python api using flask. First of all my d3 javascript code looks like this:

var country_search = d3.select("#submit")

country_search.on("click", function(){
    d3.event.preventDefault();

    var user_input_country = d3.select("#user_country").node().value;
    d3.select("#user_country").node().value = "";

    clean_format(user_input_country, function(cleaned_input){

        d3.json(`/cities/${cleaned_input}`)
            .then(function(response){
            console.log(response);
            map = d3.select(".map-container")
            map.selectAll("div").remove();

            CreateMapTag();


            setTimeout(function(){ markermap(response) }, 1000);
        });
        d3.json(`/nlp/${cleaned_input}`)
            .then(function(summarized_text){

            div = d3.select(".text-summarize");
            div.selectAll("p").remove();
            div.selectAll("h3").remove();

            // receive summarized text from python
            RenderTitle(cleaned_input);                
            RenderURL(cleaned_input);
            RenderTextSummary(summarized_text);
        });
    })
});

from d3.json(/cities/${cleaned_input}) .then(function(response) It pass cleaned_input and gets result from python that goes like this:

@app.route("/cities/<country>")
def cities(country):
    """ for given country
    1. get list of cities from json file.
    2. loop through each city and call api to get AQI data.
    3. store into db
    4. return {city:[], aqi:[], lat_lng:[a,b]} so heat map can be made.
    """
    with open('static/db/final_cities_countries.json') as f:
        data = json.load(f)

    # if data exists grab data, if not query again and store into db, then grab data
    query = db.select([Aqi]).where(Aqi.Country == country)
    result = db.engine.execute(query).fetchall()
    if len(result) < 1:
        list_of_cities = data[country]
        for city in list_of_cities:
            # store into db if aqi_call is ONLY successful with status:ok
            if get_aqi(city) != None:
                aqi_response = get_aqi(city)
                time = aqi_response['data']['time']['s']
                try:
                    lat = aqi_response['data']['city']['geo'][0]
                    lng = aqi_response['data']['city']['geo'][1]
                except:
                    continue # skipping city that have no defined coordinates.

                # Top 5 pollutants
                try:
                    o3 = aqi_response['data']['iaqi']['o3']['v']
                except:
                    o3 = -1
                try:
                    so2 = aqi_response['data']['iaqi']['so2']['v']
                except:
                    so2 = -1
                try:
                    no2 = aqi_response['data']['iaqi']['no2']['v']
                except:
                    no2 = -1
                try:
                    pm25 = aqi_response['data']['iaqi']['pm25']['v'] #pm2.5
                except:
                    pm25 = -1
                try:
                    co = aqi_response['data']['iaqi']['co']['v']
                except:
                    co = -1

                # creating instance of Aqi class(row in MySQL table) and inserting into aqi table
                insert_to_db = Aqi(country, city, aqi_response['data']['aqi'], o3, so2, no2, pm25, co, lat, lng, time)
                db.session.add(insert_to_db)
                db.session.commit()

        # this time it will have more  
        query = db.select([Aqi]).where(Aqi.Country == country)
        # result = [(id, country, city,...), (id, country, city,...), ...etc.]
        result = db.engine.execute(query).fetchall()

    # sending back list of dictionaries. [{id:x, country:y, city:z, etc...}, {},{},...]
    return jsonify([dict(row) for row in result])

I've originally used d3.json v4 which does not use promise however read about the problem and changed to v5 and still doesn't work.