The T4A may have been issued for your Post Secondary in 2017 because that is when you received your funding (scholarship/grant/bursary). If you are considered a full-time qualifying student for 2017 or 2018, post-secondary school scholarships, fellowships and bursaries received are not taxable up to the total amount required to support you in the program.
It's important to enter this amount from Box 105 in the correct box in TurboTax to ensure you get the right exemption and all the income is not included as taxable to you.
https://turbotax.intuit.ca/tips/understanding-tuition-tax-credits-6549