I am doing taxes from 2021 - small business revenue with expenses (student) and tuition to add. When I add in my tuition, it goes from a refund to owing. Why is this?

What likely happened is that when you put in your income, TurboTax claimed the Canada Workers Benefit (CWB) (aka the Working Income Tax Benefit) for you. This is a refundable tax credit for people who work, but don’t make very much. However, students are usually not eligible for this credit, so when you added your tuition, then the CWB was removed, lowering your refund.