Redeeming Miles Ticket

I recently redeemed miles for two tickets to MSP. The website took the miles and issued the reservation/ticket but says that it can't assign me a "complimentary" seat(s) at this time. However, it does say I can purchase some seats for some additional dollars, which I have not done.

Does this mean I won't get a seat when I get to the airport unless I pay this extra stipend?

Thanks,

M8