Hi,
There are a few things you can try.
You can switch seating positions and see if that helps.
You can try twisting the buckle stalk down up to 3 full turns to see if changing the length of the buckle stalk moves the loop out of the way.
If there's room you can try scooting the seat a little sideways (but not over the edge or on top of the buckle or belt anchors) to see if that helps.
If that doesn't work, the seat is likely incompatible with seatbelt in your car. If your child is under the maximum weight for lower anchor install, you can do that for now. You could call the company and see if they have any other advice. Unfortunately, not every seat works in every vehicle.
Good luck!