Python app runs successfully on local machine but gives error "Internal Server Error" when deployed to Heroku

Question

I've written a pretty simple Python application using Pandas & Flask that looks like this:

import mimetypes
import pandas as pd
from flask import Flask, render_template, request, redirect, url_for, send_from_directory, send_file
import os


app = Flask(__name__)


@app.route('/')
@app.route('/home')
def home():
    return render_template("index.html")


@app.route('/result', methods=['POST', 'GET'])
def result():

    if request.method == "POST":

        pd.set_option('display.max_rows', None)
        pd.set_option('display.max_columns', None)
        pd.set_option('display.width', None)
        pd.set_option('display.max_colwidth', None)

        csv_file = request.files['file']
        filename = csv_file.filename
        csv_file.save(os.path.join("uploads", csv_file.filename))

        df1 = pd.DataFrame()

        df2 = pd.read_csv("./uploads/pipeline_export.csv")
        df1 = pd.concat([df1, df2])

        df1['Name'] = '=HYPERLINK(' '"' + df2['Profile URL'] + '"' + \
            ',' + '"' + df2['First Name'] + ' ' + df2['Last Name'] + '"' + ')'
        df1.insert(0, 'Full Name', df1['Name'])
        del df1['Last Name']
        del df1['First Name']
        del df1['Name']
        del df1['Profile URL']
        df1 = df1.fillna('')
        # df1.to_excel("C:\\Users\\Sam\\Desktop\\convertedexport.xlsx")
        # convertedFile = df1.to_excel("converted.xlsx")
        df1.to_excel(os.path.join("./downloads", "convertedFile.xlsx"))
        return send_file('./downloads/convertedFile.xlsx', as_attachment=True)
        # return redirect(url_for('index.html', filename=filename))
        # return render_template("/index.html")


@app.route("/uploads/<filename>")
def uploaded_file(filename):
    filename_processed = 'processed' + '-' + filename
    return send_from_directory("downloads", filename, as_attachment=True, attachment_filename=filename_processed)


if __name__ == "__main__":
    app.run(debug=True)

The link to the repo can be found here: https://github.com/Web-Dev-Jr/TitusConvertPy

It works fine locally but when I deploy to Heroku I get the following error:

Internal Server Error

The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.

Has anyone else run into something like this and could explain what my issue is?