When the end date for an opportunity arrives or has passed, you cannot make changes or add questions to an apply-to opportunity. 
  • Select Opportunity>Portfolios>Opportunity, and locate the opportunity in question.
  • Click View.
  • You will land by default on the Details tab.
  • Scroll to the dates area and note the End Date for the opportunity. 
  • If it has passed, you cannot make changes to the questions.

Learn more about what you can do now:
What Happens When the End Date for an Opportunity Arrives?