namespace "Eigen" has no member "Vector3f" ERROR

I'm using UTM on a MacBook to run a virtual machine to learn about computer graphics.

As I'm new to cpp and , I got this error when including it and fixed it by

#include <eigen3/Eigen/Eigen>

but there's another problem here:

Eigen::Vector3f a() const { return v[0]; }Eigen::Vector3f b() const { return v[1]; }Eigen::Vector3f c() const { return v[2]; }

wherever there is Eigen::xxx, there's an error of namespace "Eigen" has no member "xxx".

How could I fix this? and why would this happen?

VM version: Ubuntu 22.04.4 LTS on Macbook Pro M2

GCC version: 11.4.0

Thanks in advance.