How to add Contact "Notes" field in webform?
Here's a similar question: Can I add a space for people to write notes: like dietary needs and or comments?

And here's an outstanding request for the feature here: https://www.drupal.org/project/webform_civicrm/issues/2320295

The conclusion is: consider putting the note somewhere else, e.g. an activity or custom field.

Although the notes field seems like the right place, it's not a bad idea to NOT put it there, since the notes field tends to be private for administrators and hiding the contact's notes in there means they might not get seen.

If you work forwards and think about what those notes are for, that might suggest a better location. A custom field seems like the right answer.