You might encounter the classic command not found

line 1: yarn: command not found

You can visit the information of PATH variable in Manage Jenkins > System Information and found that on Mac homebrew path is not there

Adding Path with Plus Sign

Now you fix this by going to Manage Jenkins > Configure System and looking for environment variables then tick it up.

Adding PATH+HOME_BREW there, note that you CANNOT override this just like the .bashrc style of export $PATH:/your/own/path

Hope this help !