Acceptance tag not showing error message

I add to my contact form a little check box for the terms and condition using acceptance tag
[acceptance acceptance-111]

It works because if I check the box I’m able to send the message, if not the message will not be sent.

BUT:
If I don’t check the box
1) the SUBMIT button is still clickable. It means that I can click on it but simply nothing happen
2) there is no error message. If I don’t check the acceptance checkbox and click the SUBMIT button I should get something like Please accept the terms to proceed., but nothing happen.

I also tried to add acceptance_as_validation: on in this way:

[acceptance acceptance-111 acceptance_as_validation: on]

But nothing happen.

Am I doing something wrong? Is it a plugin problem or could be some interaction problem with my current theme?

Thanx

https://wordpress.org/plugins/contact-form-7/