Carla is a morphed figure. She cannot wear standard V3 clothes.

You have to be careful choosing characters and clothing. Some clothing has lots of morphs, some has none. If your character is morphed, the clothing must have the same morphs built-in, or it won't fit.

If a character has custom morphs, as Carla does, fitting clothing to her can be a real problem. Generally, you have to use a program such as The Tailor to transfer morphs from the character to the clothing. The ReadMe for Carla says he's provided some clothing morphs for her. You will need a free program called Morph Manager to transfer them to the clothing you want to use.

But if you're really new to Poser, you might be better off using a standard character, before you try a morphed one. You can set the Carla morphs to zero, so she has V3's standard body. Then V3 clothes will conform properly.