[Answered ]-Django Form – Select a valid choice. That choice is not one of the available choices

This is due to the way Django validates forms.

To put it simply, it does not expect to be answered a choice that it did not generate.

In the docs :
https://docs.djangoproject.com/en/1.11/ref/forms/fields/#choicefield

Validates that the given value exists in the list of choices.

A simple “Django ajax choice form” search on this site will return you some possible workarounds.