Sounds like apache or some other web server might already be running on that port.
ps aux | grep apache
If you cannot answer these questions yourself then I suggest you install virtualbox on your local machine and get nodebb working locally but following the docs below. Once you are successful with this, you will have learned alot and can then try and deploy on Google infrastructure.
I know you want to deploy straight to production but my way will be quicker in the long run. The next question you will have will be regarding NGINX and how to get it running on port 80. The answers to all your questions are in the docs. The docs are very easy to follow.
I can create the website by using 127.0.0.1 IP but the connection is refused by google's internal IP
(127 is a local ip, you need to provide your internet IP here)
"url": "I typed here my domain which is connected to Google's external IP",
"secret": "the key",
"host": "Google's internal IP", (should be 127.0.0.1)
"port": "I couldnt find this out?", (4567)
"username": "Is this the nodebb user or the user I created for admin database?", (nodebb user)
"password": "Password", (mongo password for Nodebb database)
Not sure if they will help but its a start