Have you checked your T4 for an amount in box 40? If box 40 is not filled out, you might try to check if it was included with your income on Box 14. Could it be possible that the T4 was not accurately filled out?
From the information you provided, your professional dues appear to be a taxable benefit. Review the information here: https://turbotax.intuit.ca/tips/how-to-claim-union-dues-on-your-tax-return-5496