I am a US citizen preparing T4, and it will not let me put my US SS# in place of SIN. I was a US international student attending UWO. Any idea on why?
You need to have a SIN number in order to file a tax return in Canada. This Canada.ca website has info on how to apply for one: https://www.canada.ca/en/employment-social-development/services/sin/apply.html
