I'm trying to update the document but the error says the query has already been executed.

MongooseError: Query was already executed: footballs.updateOne({ date: 'January 4' }, {})

app.post('/api/bookslot', async (req, res) => {
  console.log(req.body);
  try {
    const token = req.headers['x-access-token'];
    const decoded = jwt.verify(token, 'secret123');
    const email = decoded.email;
    const user = await UserModel.findOne({ email: email });

    let sportname = req.body.selectedSport.toLowerCase();
    const time = req.body.slotTime;
    const seats = req.body.availableSeats - 1;

    if (!sportname.endsWith('s')) {
      sportname = sportname.concat('s');
    }
    const NewSlotModel = mongoose.model(sportname, slotSchema);
    var update = {};
    update[time] = seats - 1;
    console.log(update);
    const a = await NewSlotModel.updateOne(
      { date: req.body.slotDate },
      { $set: update },
      function (err, success) {
        if (err) return handleError(err);
      }
    );

    return res.json({ status: 'ok' });
  } catch (e) {
    console.log(e);
    res.json({ status: 'error' });
  }
});

where am I going wrong?

You are using both async/await and callbacks in your code, causing mongoose to throw an error. The actual effect of using them both is exactly the error type that you are receiving:

Query was already executed

Mongoose v6 does not allow duplicate queries.

Mongoose no longer allows executing the same query object twice. If you do, you'll get a Query was already executed error. Executing the same query instance twice is typically indicative of mixing callbacks and promises, but if you need to execute the same query twice, you can call Query#clone() to clone the query and re-execute it. See gh-7398

Duplicate Query Execution

To fix the issue, just remove the third argument from the await

NewSlotModel.updateOne

Making it:

const a = await NewSlotModel.updateOne(
  { date: req.body.slotDate },
  { $set: update }
);