I can still select multiple options in a checkbox field whe the maximum is set to 1

I've just tried this but I can still check both checkboxes. This pertains to the optional question: "This contribution is a tribute." Here's a link to the form:

https://form.jotform.us/JFedGMW/donation-form

 I've already set the limit to a maximum of 1.

Though it checkbox fields will allow for selecting multiple options when the maximum is set to 1, there will be an error given when doing so:

As long as this error is active, the form will not allow for completing the submission.

Thanks for the clarification.