Line 15 on Form T777S is your employment income from box 14 of your T4.
Did you say “Yes” to the question “Will you claim employment expenses for working at home due to COVID-19 for income on this T4?” on your T4?
If not, please click on Find (magnifying glass) in the top right-hand corner. Type "T4" in the box, then select "T4 Employment Income" from the list and click “Go”.
Click “Edit” next to the T4 that you want to change. You will find the question “Will you claim employment expenses for working at home due to COVID-19 for income on this T4?” at the bottom of the page. Select “Yes”, then click “Save and Continue” to save your changes.