Seatpost clamp sizing issues?
First double check that it's not a simple case of it just being tight, which is actually pretty common. Remove the bolt completely and gently wedge it open just a little bit with a screwdriver, and see if it will fit on then. Do not force it - you're checking to see if it fits correctly albeit a bit snugly.

If that doesn't work, and/or if it's just clearly the wrong size completely, you need to measure both the clamp you've got and the outside diameter of the seat tube. Ideally use a caliper. You could almost get away with just using a ruler to see what size the frame is actually "trying to be", but the problem is that frames do exist just .5mm larger than 34.9, even though 34.9 (aka 35mm) is much more common.

There are some weird, semi-proprietary diameters in the 35-36mm range that have given rise to clamps like this and this. 36.4mm is also a standard size, but as far as I've observed it's not common for carbon road bikes.