What is happening is your tax return was originally showing a refund of the Canada Workers Benefit (CWB) that full-time students are not eligible for. As soon as you enter your T2202A, the credit is removed and your refund is reduced.
Based on your employment earnings the software calculates your entitlement to the CWB, however, the government disallows this benefit to students. So entering the T2202 Tuition Certificate slip causes a recalculation and removal of this amount from the refund.
More information on the CWB can be found here: Canada Workers Benefit (Working Income Tax Benefit before 2019).
Without seeing your entire tax situation it's hard to tell from this forum why you are owing money after entering all your slips, but one thing you should check is that you have entered your T4A in the correct box 105. You have to choose according to your attendance in school in order for that income to be calculated correctly and receive any "scholarship exemptions" you are entitled to.
Please see this link for more information: Line 13010 – Scholarships, fellowships, bursaries, and artists' project grants
