2
There are two errors in the code shown in the question.
It's only the form builder that can add options to form elements. Differently, you will get the error you quoted.
An invalid selection has been identified. Please contact the site administrator.
What the AJAX callback does is simply returning the form element that needs to be changed. In your case, ...
1
The first point is the only straightforward way. A regular node edit form has already Ajax form elements. You can check the "ajax" subarray of drupal-settings-json delivered to the browser.
You should see your own Ajax callback here as well:
"ajax": {
"edit-field-article-add-more": {
"callback": [
&...
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