I upgraded my Ghost Blog server’s OS from Debian 10 to 11, but the site gives a 502 Bad Gateway and in the command line.
When I go to /var/www/ghost and run ghost check update, I get the following error as well /usr/bin/env: ‘node’: No such file or directory
Do you have nodejs installed, and if so, what version are you currently running?
When I try node in the command line, it says command not found. It looks like upgrading Debian removed node.
Ok in the mean time I’m going to do a restore from back up which is easy and will get the site back up quickly.
Then I might have to run fresh installation and see about transferring over the data to the new server.
Yes, that’s what I suspected. Please mark my response as a solution if this solves the problem for you, so that others might find this helpful as well 
Done! Thanks for your help! That cuts down the troubleshooting checklist significantly.
Edit: I restored the site back in the meantime.