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I think the one you're referring is the "Form Warning" message. Please follow the Steps below for your info.
1. Click “Preferences”.
2. Click “Advance Settings”.
3. Click “Edit Form Warning”.
4. Click and change “This field is required.” to “Whoops! looks like you missed something!”
5. Example: http://form.jotform.me/form/50443315634449
These steps provided by rsalvo will allow you to change your form warning messages.
Please feel free to contact us if you need further assistance.