Internal Server Error 500 from API when deploying front-end React.js backend Express app

Question

For my senior capstone, my group and I have developed a web-based application to simulate Bitcoin - using react.js for the front-end and node.js/express for the back-end. Up until recently, we've had all of simulation-creating-code (javascript files) inside the src directory, meaning it was being built client-side. Due to high waiting times to create a simulation from all the hashing necessary in transactions, we decided that our simulation-creating-code would be better suited for the back-end rather than the front end. Taking the load off the client and putting it on the server drastically improved the speed of creating a simulation, so 'Great success!'.

When we made this change, we ended up having some issues with require and import statements. Reactjs only supports import statements and Express uses require statements. We had to use some js functions that we developed in our API's so we imported them with require statements, and we thought we thought it was resolved because on our development environment, everything runs as smooth as butter, but once it's deployed, our login page is unable to make an API call. The error is: Failed to load resource: the server responded with a status of 500 (Internal Server Error).

It's interesting because this route in the API worked prior to making this big switch from require to import, and those changes were in other files/routes. The login API remains completely unchanged.

Either way, I'll drop some code in case it's helpful in troubleshooting. server.js

const express = require("express");
const app = express();
const router = express.Router();
const path = require("path");
var cors = require("cors");

require("dotenv").config();

app.use(express.json({ limit: "50mb" }));
app.use(express.urlencoded({ limit: "50mb" }));

app.use(function (req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header(
    "Access-Control-Allow-Headers",
    "Origin, X-Requested-With, Content-Type, Accept"
  );
  next();
});

// List of routes
router.use("/api/users", require("./api/users"));
router.use("/api/data", require("./api/data"));
router.use("/api/share", require("./api/share"));
router.use("/api/addresses", require("./api/addresses"));

const root = path.join(__dirname, "client/build");
app.use(express.static(root));

app.use(router);
app.use(cors({ origin: true, credentials: true }));

app.listen(
  process.env.PORT,
  () => `Server running on port ${process.env.PORT}`
);

api/users.js login route

const express = require("express");
const app = express();
const db = require("../dbConn");
const bcrypt = require("bcrypt-nodejs");
const cors = require("cors");
const router = express.Router();
const jwt = require("jwt-simple");
const config = require("../configuration/config.json");

// to parse JSON
app.use(express.json());

router.post("/login", (req, res) => {
  //check if email and password are sent
  if (!req.body.email || !req.body.password) {
    return res.status(401).json({ error: "Missing username and/or password" });
  }
  // go into mysql and get info
  let qry = `select * from user where email = "${req.body.email}"`;
  db.query(qry, (err, rows) => {
    if (err) {
      return res.status(500).json({ error: err });
    }
    // assert: no error - process the result set
    if (rows.length == 0) {
      // no users found
      res.status(400).json({ msg: "No users found" });
    } else {
      // process the user records
      let users = [];
      rows.forEach((row) => {
        let user = {
          uid: row.uid,
          email: row.email,
          role: row.role,
          dateCreated: row.created_date,
          password: row.password,
        };
        users.push(user);
      });
      if (users[0]) {
        // Does given password hash match the database password hash?
        bcrypt.compare(req.body.password, users[0].password, (err, result) => {
          // Send back a token that contains the user's username
          const token = jwt.encode({ email: req.body.email }, config.secret);
          if (result == true) {
            res.status(200).json({
              msg: "user authenticated",
              fname: users[0].fname,
              lname: users[0].lname,
              role: users[0].role,
              token: token,
            });
          } else {
            res.sendStatus(401);
          }
        });
      }
    }
  });
});

router.post("/auth", cors(), (req, res) => {
  try {
    let user = jwt.decode(req.body.token, config.secret);
    res.status(200).send(user);
  } catch (err) {
    res.sendStatus(401);
  }
});

SignIn.js client/src/components. This is wrapped in a react.useEffect() arrow function, but again I don't believe the issue is here because this page remains unchanged from a working version.

const handleSubmit = (e) => {
    e.preventDefault();

    const credentials = { email, password };

    // API call to login to account
    // if successful, redirect to landing page
    // if not, display error message
    fetch(`http://${process.env.REACT_APP_API_URL}/api/users/login`, {
      method: "POST",
      headers: {
        "Content-Type": "application/json",
      },
      body: JSON.stringify(credentials),
    })
      .then(async (res) => {
        if (res.status == 200) {
          return res.json();
        } else {
          throw new Error("Failed to Login!");
        }
      })
      .then(async (res) => {
        // Store token in cookie
        setCookie("token", res.token, { path: "/${path}", maxAge: 3600 * 24 });

        // Toggle state of sign in
        toggleSignIn();

        // Feedback
        setFeedback(true);
        setFeedbackObj({ message: "Signed in!", severity: "success" });

        //redirect
        history.push(`${process.env.PUBLIC_URL}/simulation`);
      })
      .catch(async (err) => {
        // Feedback
        setFeedback(true);
        setFeedbackObj({ message: "Sign In Error", severity: "error" });

        console.error(err);
      });
  };

If there are any other files that are of interest please let me know.

I've tried to mess with the proxy in package.json, but I don't think thats the answer because it was working previously. I've had a really difficult time finding others with similar issues or resources other than how to build a simple app with Express backend and React.js front end. This is not our issue because our application was working perfectly before this big switch. I believe the issue is stemming from require statements in our API and the running of JS functions in the API. I have no way to confirm this because in production (deployment), the errors are super uninformative, and in development, it runs perfectly fine.

I have been trying to solve this issue for a couple of weeks now, and I've made very little progress. If anyone has suggestions or tips on troubleshooting deployment, I would greatly appreciate it.

Thanks!