Thanks to @siggemannen I was able to solve the problem. Seeing the error made me think about whether a connection was assigned and I inspected other people's code and realized that when creating a PreparedStatement I needed to pass in the ConnectionPool object from mssql's connect function.

So I made these changes.

// db.js
var pool;

sql.connect(dbConfig).then(result => result = pool).catch(err => {
  console.error("Connection issue: " + err)
  console.error(err.stack)
})

module.exports = { pool, ...}

Then in the file where I needed to use the PreparedStatement

const { pool } = require("db");

// ... code

router.get('/debtors/', async (req, res) => {
  const column = req.query.column
  const ps = new sql.PreparedStatement(pool) // <--- This is the important part
  ps.input('param', sql.VarChar(200))
  ps.prepare('select * from debtor WHERE first = @param', err => {
    console.log("ps.prepare err: " + err)

    ps.execute({ param: column }, (err, result) => {
      console.log("ps.execute: " + err);

      res.send(result.recordset);
      
      ps.unprepare(err => {
      })
    })
  })
})

The problem was that since I assumed that the callback activated it meant that there was no error. Turns out the callback activates regardless of whether there is an error or not, and to check if there is an error. I need to check if err !== undefined if it isn't undefined then there's an error message in there.