Yes, actually I solved the problem. I will leave the answer in case somebody is having the same problem.
Turned out I was entering the server from Google Cloud Platform -> Compute Engine -> Activate Google Cloud Shell (the icon to the right). That's wrong.
You have to access it from GCP -> Deployment Manager -> Deployment -> click your server -> Where is "Get started with OpenCart Certified by Bitnami" click on SSH.
Thank you for the reply anyway. All best!