When creating a cycle, you can define if each question is mandatory or optional. This might be useful if you think not every contributor will be able to answer every question.
To do so, choose 'Score optional' and/or 'Comment optional' from the right-hand dropdowns within the question / skill settings. If you create a custom question, selecting 'Score optional' or 'Score required' will present your default rating scale for that question (which can be customized for each question, please see here).
As a consequence of making a score optional, employees filling out a review can choose 'Not applicable' (N/A) when they feel they cannot answer the question.
If you choose 'Score required,' the question will be mandatory and cannot be skipped.
You can always come back to these settings later and change them - even after the cycle kickoff. This makes it possible to activate the N/A option later in the process (e.g., when some of the contributors tell you they found it hard to answer every question). If you do so, the changes will be active immediately in all questionnaires, even those already started.