Symfony choice field type reports "This value is not valid" when submitting an invalid option. How do I change this?

The sentence "The value is not valid" is the default error message which results of a form field transformation failure. This error message is related to each field in case of a data transformer failed (in your case it seems you try to send an invalid choice value). If you want to override it, you can use the invalid_message & invalid_message_parameters form type options.

I can't find the official documentation of this property right now but you can take a look to this for more details: link

The message is coming from the choice constraint, not the choice field type class.

http://symfony.com/doc/current/reference/constraints/Choice.html

The message option can be overridden with your own message.