On the submit button of your form, click the checkbox for Custom Validation on the Functionality tab. This will allow you to specify a model to determine validation. Your model should look like the 2nd picture (approximately):
If checkbox checked (True False Rule) and no text in textbox (Get Text Length of ThisFormData.textoutput + Number Range Rule) throw a fatal rulebreak (Create Value option of 1st picture) with the error text in "Name" in the End component
Else return a null rulebreak in your End component (Null Value option of 1st picture)