分析：因为安装的ubuntu版本是20.04，对应安的noetic包里自带的是Opencv4.2，因此需要更改ros编译时使用的opencv版本。

解决：打开/opt/ros/noetic/share/cv_bridge/cmake/cv_bridgeConfig.cmake文件，做以下更改：（注释掉的是源代码，后边是更改后的）

#if(NOT "include;/usr/include/opencv4 " STREQUAL " ")

if(NOT "include;/usr/local/include/opencv; /usr/local/include/opencv2 " STREQUAL " ")

set(cv_bridge_INCLUDE_DIRS "")

#set(_include_dirs "include;/usr/include/opencv4")

set(_include_dirs "include;/usr/local/lib;/usr/local/include/opencv;/usr/local/include/opencv2;/usr/local/include;/usr/include")

之后再安装一个Opencv 3.x的版本即可。（可参考链接安装：https://blog.csdn.net/qq_36351159/article/details/108255033）