MAMP PRO is saying another process is running on the port I want to use
MAMP PRO may not be able to run on the web server, database or other port you want to start a service on. you may see an error message indicating that the port is in use. MAMP PRO will automatically suggest a different port.
To confirm that there is another process running on a port you want to use, type the following into the terminal.
sudo lsof -i :80 # checks port 80