There were three problems:

• you were missing a lot of curly braces for the if statements (note: apparently these aren’t required, but I prefer them for readability)
• you need to put document.getElementById('image_info').reportValidity(); after the setCustomValidity
• you weren’t sending any parameters to imageErrorMessage

function imageErrorMessage(image_info){
  if(image_info == "") {
    document.getElementById('image_info').setCustomValidity('Please enter your information.');
    document.getElementById('image_info').reportValidity();
  } else {
    document.getElementById('image_info').setCustomValidity('')
    document.getElementById('image_info').reportValidity();
  }
}

<p>
  <label for="image_info">Information</label>
  <br>
  <input type="text" name="image_info" id="image_info">
</p>
<br>
<button type="button" onclick="imageErrorMessage(document.getElementById('image_info').value)">Check</button>

Some errors in your code:

• The HTML was in a paragraph instead of a <form>.
• You were not validating the field correctly
• You were not preventing submit of the form
• You were not presenting the error message to the user

My solution:

• Has a <form>
• Prevents submit of the form
• Checks the field
• Gives an error if empty, otherwise submits the form via JavaScript

const $form = document.getElementById('form')
const $submit = document.getElementById('submitButton')
const $imageInfo = document.getElementById('image_info')

$submit.onclick = (event) => {
  event.preventDefault()

  if (!$imageInfo.value) {
    $imageInfo.setCustomValidity('Please enter your information.')
    $imageInfo.reportValidity()
  } else {
    $form.submit()
  }
}

* {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';
}

<form id="form">
  <label for="image_info">Information:</label>
  <input type="text" name="image_info" id="image_info">
  <button id="submitButton">Submit</button>
</form>

I am trying to implement a button in HTML that calls a function on click, which checks whether a textinput is empty. If so (the information hasn’t been entered), an error message for the input field should be generated.

I am experimenting with a button that doesn’t submit the form. However, if you have a solution with a button that does submit the form (unless the string is empty), I’ll gladly take it. In that case (and in any really) I would like to work with setCustomValidity, as I want an error message right away and not after the page reloads if that makes sense (because then the input in the form isn’t kept).

This is what I have so far:

<p>
        <label for="image_info">Information</label>
        <br>
        <input type="text" name="image_info" id="image_info">
    </p>

    <br>

    <button type="button" onclick="imageErrorMessage()">Check</button>

    <script type="text/javascript">
        function imageErrorMessage(image_info){
        if(image_info === "")document.getElementById('image_info').setCustomValidity('Please enter your information.');
        else document.getElementById('image_info').setCustomValidity('')
        }
    </script>

Unfortunately something seems to be missing/wrong, as it doesn’t work. I’m fairly new to Javascript so the mistake could be crystal clear and I wouldn’t know.

Thanks very much!