All you have to do is to push below line inside the addEventListener. You are caputuring or taking value at the start of the script, till then the input is empty and then the value will be empty. So I think you want to take a value when you enter something in the input and when you click calculate button. i.e. inside the event listener.

calculateTipBtn.addEventListener('click', () => {
  const totalBill = document.getElementById('totalBill').value; // THIS
  if (totalBill.value === '') {
    console.log("Value can't be empty");
  } else {
    alert('there is something');
    console.log(totalBill);
  }
  console.log(totalBill);
});

const calculateTipBtn = document.getElementById('calculate');
const resetBtn = document.getElementById('reset');
const totalBill = document.getElementById('totalBill');

calculateTipBtn.addEventListener('click', () => {
  const totalBillValue = totalBill.value;
  if (totalBill.value === '') {
    console.log("Value can't be empty");
  } else {
    alert('there is something');
    console.log(totalBillValue);
  }
});

resetBtn.addEventListener('click', () => {
  window.location.reload();
});

<div id="wrapper">
  <header>Vanilla Tip Calculator</header>

  <div class="content">
    <label for="totalBill">Enter Total bill</label>
    <input type="text" name="total" id="totalBill" />
    <label for="tipPercentage"> How much tip are you feeling?</label>
    <input type="checkbox" name="tip" value="15" /><span>15%</span>
    <input type="checkbox" name="tip" value="20" /><span>20%</span>
    <input type="checkbox" name="tip" value="30" /><span>30%</span>
    <br />
    <button id="calculate">Calculate!</button>
    <button id="reset">Reset</button>
  </div>

  <div class="tip-result"></div>
</div>

I think you need to move

const totalBill = document.getElementById('totalBill').value;

Inside the on click event, otherwise you will read the empty value when the page loads.

You initialize totalBill variable on script load and before you click on the calculate button. move totalBill inside onClick

const calculateTipBtn = document.getElementById('calculate');
const resetBtn = document.getElementById('reset');

calculateTipBtn.addEventListener('click', () => {
  const totalBill = document.getElementById('totalBill').value;
  if (totalBill === '') {
    console.log('Value can\'t be empty')
  } else {
    alert('there is something')
    console.log(totalBill)
  }
})

resetBtn.addEventListener('click', () => {
  window.location.reload()
})