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I did check the "Minimum Selection" and "Maximum Selection" of the checkbox question and it seems to work as expected. Please check the screenshot below:
Please be noted that you will be able to check all the checkboxes but you will not be able to submit the form as it will trigger the validation error as displayed in my screenshot above.
I did check your last edited form and it seems to be working fine. Id does display the validation error message. Please check the screenshot below:
Have you embedded this form in your web page where it is not working? Please share your web page URL and we will take a look.
We will wait for your response.