-- Could NOT find PY_em (missing: PY_EM)
CMake Error at /opt/ros/noetic/share/catkin/cmake/empy.cmake:30 (message):
  Unable to find either executable 'empy' or Python module 'em'...  try
  installing the package 'python3-empy'
Call Stack (most recent call first):
  /opt/ros/noetic/share/catkin/cmake/all.cmake:164 (include)
  /opt/ros/noetic/share/catkin/cmake/catkinConfig.cmake:20 (include)
  CMakeLists.txt:58 (find_package)

-- Configuring incomplete, errors occurred!

这是因为ros和anaconda冲突所致。所幸20.04系统的ros-neotic已经支持了python3，因此安装：

conda install -c conda-forge empy

即可。

上面那种方式如果不生效的，每次编译的时候，使用下面的方法也可以：

catkin_make -DPYTHON_EXECUTABLE=/usr/bin/python3