The issue is definitely that you are usingzsh as your default shell. On line 178 of the baxter.sh script, you can see they start a new shell using the SHELL environment variable with the argument --rcfile. zsh doesn't have this argument available, and that is what triggers your error. If you want to use this script as-is, you can't use zsh. If you want zsh and this script, then you'll need to debug the script a bit
Just FYI, I personally never use the baxter.sh script when working with Baxter. All the script really does is ensure that you have set the ROS_MASTER_URI and the ROS_HOSTNAME/ROS_IP environment variables correctly. I manage these variables with my own scripts and avoid their script primarily because