Error "Uncaught TypeError: Cannot set property 'value' of undefined" on submit form

I'm getting the following error when trying to submit a form using Joomla.submitbutton('book.submit')

The error I get is:

Uncaught TypeError: Cannot set property 'value' of undefined
at Object.Joomla.submitform (core-uncompressed.js?f096f6025f0c22829d968a68944d3145:66)
at Object.Joomla.submitbutton (submitbutton.js:26)
at HTMLButtonElement.onclick (gallery:195)

submitbutton.js line 26:

if (isValid)
{
    Joomla.submitform(task);
    return true;
}

core-uncompressed.js?f096f6025f0c22829d968a68944d3145 line 66:

  if (task) {
        form.task.value = task;
  }

It means that, somehow, the button is empty or something related to this but I do not understand why.

1. It was perfectly working and now it does not work.

2. The code has not changed at all.

3. I have not touched the project in the past 5 days.

4. It was working when I last checked.

When I went to continue where I left off, I ran into this unpleasant issue.