Hi Caroline! Thanks for the question.
Here's one solution: add a trigger to the next button that only jumps to the next slide IF the value of username variable is NOT equal to a blank value.
To do that, add a new trigger to the next button or submit button the user clicks after they enter their name. At the bottom of the Trigger click "Show Conditions" and add a condition. From List make sure "Variable" is checked, and from the drop down with variables select your username variable. Next, under "operator" choose "NOT equal to", under type leave "Value" and then in the Value field, leave it blank.
Once you do this, the submit or next button will not work so long as the text entry field is blank. There has to be at least one value (letter) in it when the user clicks Next or Submit, to be able to move along to the next slide. Hopefully this helps and you understand the directions. Let me know if you need further help!