If any one saith, that the commandments of God are, even for one that is justified and constituted in grace, impossible to keep; let him be anathema.
There are Protestants who claim that it is impossible for anyone to obey God's Law perfectly. These people are mistaken, and this canon of Trent is intended to warn Catholics against such a lie.
This canon is not contrary to justification by grace alone. As we've seen repeatedly, we cannot justify ourselves, because we cannot redeem ourselves from our sins by anything that we might do. So those who suppose that this canon opens some sort of possibility for justifying oneself by his action are wrong.