Ah, I just solved it. It seems I needed to do this instead:

app.get("/api/data/:serialNumber", async (req, res): Promise<any> => {

Here is the whole file as it stands now:

import express from "express";
import axios from "axios";
import { BEARER_TOKEN } from './config';
import { URL } from './config';

const app = express();
const PORT = 3000;

app.use(express.static("public"));

interface SerialNumberParams {
    serialNumber: string;
}

app.get("/api/data/:serialNumber", async (req, res): Promise<any> => {
    try {
        const serialNumber = req.params.serialNumber;

        if (!serialNumber || serialNumber.length < 5) {
            return res.status(400).json({ error: "Invalid serial number" });
        }

        const API_URL = `${URL}${serialNumber}/events?page=1`;

        const response = await axios.get(API_URL, {
            headers: { Authorization: `Bearer ${BEARER_TOKEN}` }
        });

        return res.json(response.data);
    } catch (error: any) {
        return res.status(500).json({ error: error.response?.data || "Error fetching data" });
    }
});


app.listen(PORT, () => {
    console.log(`Server running at http://localhost:${PORT}`);
});