Not sure what you are doing, but I never had an issue getting seat assignments on aircanada.com. Including flights on LH. This said, it happens that for low business fares, LH legs may end up in economy, and then even for Y fares, there is a fee for advanced seat assignment.

Aand yes, for international departure from YYZ even in low business class (P fares) on an AC-operated flight you have access to the Signature suite. Not so if on an LH-operated flight. Even if booked on Aircanada.com, and even if booked as a code-share.