Those errors are usually caused by your path being too long. You’ll probably see errors to this effect earlier in the CMake log.
As mentioned in the Prerequisites section of the Developer Setup instructions, you must enable Long Paths on Windows.
Even then, you might find that C:\Users\xxx\GitLab\CesiumNativeUnityDevelop\cesium-unity-samples is too long a path. Move cesium-unity-samples somewhere with a shorter path and you should have more success.
This is unfortunately a limitation of Windows and various tools running on Windows, and not something we can fix in Cesium for Unity.