The only mandatory thing for your citizenship application in regards to taxes is to completely complete question 12 which asks for your SIN and when you filed your taxes. If you did not file taxes, say "No" to Taxes filed.
Now all that said, if you say you were required to file (which you are not always) and did not, It might delay your application, as per "income tax filing requirement and physical presence requirement of the Citizenship Act". But you should still answer Q12 completely and honestly.