The tire clamps down to the part circled in red, in the and the part of the bike between the pedals goes on the post being pointed to with the yellow arrow.
I have no experience with the rack, but it's possible that the knob is slipping where it connects to the bolt that actually secures the rack. You may have to cut off the knob to remove the rack. I would contact Thule or a local dealer for advice, though. They should be willing to help with a $400 rack.
Today is not Saturday anywhere in the world. Also, if there's a bike shop near you they may be able to help. I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881
It's a trunk-mount rack, should not be too hard to figure out. Try a Google search for 'Thule 515-5033' and look at the IMAGES results to see examples of it mounted on a car. I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881
