Erikahuer
Which Question type is on the page where you want it to be disabled until answered?
You can use this.disableNextButton() or this.hideNextButton it will then disable or hide the button. Then use this.enableNextButton()or use this.showNextButton() to enable or hide. The quantity and type of questions on the page will influence if the JS will work or not. This method can therefore become quite difficult.
It is nearly always possible to force responses to questions by including suitable response options (e.g., None of the above, others etc.). This is a much simpler answer. It is reasonable to expect respondents to answer the questions if they are paid.
Hope it helps!

Try with document.getElementById?