Can I modify my 2016 return to include a loss from liquidated stocks reported on a Summary of Security Dispositions form? It's not a T5. What line would this be? 174?

The Summary of Security Dispositions is a tailored version of the T5008 Summary. To add it to your return, you would have to fill out a Schedule 3, and if you have capital gains, this would flow to Line 127 on the 2016 T1 General.

To change your 2016 return, you would need do it through CRA’s My Account service: https://www.canada.ca/en/revenue-agency/services/e-services/e-services-individuals/account-individua...