It depends. If the only T4A slip that you received was for your self-employment, then you can just check off that you were self-employed. It will then ask if all your self-employment income is reported on a T4 or T4A slip, which you can also check off.
If you have other T4A slips not related to self-employment, then you can also check off the T4A box under the “I was Employed” section.
If you have TurboTax Live Assist & Review, here is how to talk to a Tax Expert: https://turbotax.community.intuit.ca/community/navigating-turbotax/help/i-m-using-turbotax-live-assist-review-how-do-i-talk-to-a-tax-expert/00/873764