I am hoping to reserve my seat for a flight from YYZ to MUC later this week. The ticket is with Lufthansa but the code-shared flight is actually Air Canada.

I tried to reserve my seat with AC but it wouldn't recognize my booking number. Lufthansa did recognize my booking. However, reserving a seat with them is not possible since it's an AC flight.

Am I out of luck and have to take my chance at the airport?

Any advice is appreciated.