On September 20th, 2018. See this statement in context.
September 20th, 2018 / 12:30 p.m.
Associate Assistant Deputy Minister, Trade Policy and Negotiations and Lead Negotiator of the Comprehensive and Progressive Agreement for Trans-Pacific Partnership, Department of Foreign Affairs, Trade and Development
I guess the short answer is that this amendment would effectively represent a unilateral amendment to the treaty; therefore, it would make it impossible for us to ratify the agreement if the amendment carried.
See context to find out what was said next.