The CRA considers amounts in box 20 and box 48 of the T4A slip as self-employment/business income.
If you entered a T4A slip in your return with amounts in box 20 or box 48, you must complete a Form T2125: Statement of Business Activities. Only minimal entries are required; the business name can be your name and the business address can be your home address.
You'll need to "edit" the Business Statement where your T4A income is transferred to. To do so go to: Income & Expenses > Self-Employment. Depending on which box number was entered from the T4A, "Commission Income" or "Business Income" should appear here. You can then click on the "Summary" and choose to edit that particular T2125.