Hello, I haven't found a job in Canada yet. When filling out the declaration, there was a warning that I have zero income in Canada. why did this warning appear?

If it is a Warning and not an Error, then it can be ignored and skipped. The Warning alert does not automatically mean something was done or entered incorrectly. It can be triggered if a high amount of certain credits or deductions are being claimed. Click on Review to verify that the right amount has been entered by clicking on "Applies to me". Once reviewed you can click "Done with warnings" and return to your review. 

Warnings are there just to ensure that if the situation applies to you, you can go back and make any necessary modifications.
Errors, on the other hand, will prevent you from filing because something in your return is incorrect and the return will not be accepted by the Canada Revenue Agency.

Thank you for choosing TurboTax.