Depending on your other income, you may be required to file (such as, withdrawals from a pension or 401k, or unemployment compensation, or disability or maternity pay). However, to claim any of the refundable credits, you must have "compensation" from working.
Some forms of sick pay, disability pay, or maternity leave pay may be considered "compensation", since you earned those benefits from your previous work. If you receive a W-2 for your sick pay, disability pay, or maternity leave pay, and there is a dollar figure in box 3 for social security wages, that will be considered "compensation" and may allow partial or full credits depending on your other tax situations.)
