Try rebooting the server and then trying the upgrade again. I had a similar issue when downgrading to an earlier version of Node. It’s like not all parts of the system recognise the change, even if they say they do.
It’s possible that some part of the Ghost CLI still thinks you’re running 16.13, even as it simultaneously throws an error with the correct version - 16.14 - cited.
In my case I was using NVM to indicate the appropriate version of node. That version of node would “disappear” when I logged off because the variable was only properly set for the current session.
For me, I was able to execute this command which sets the default node version in the shell:
Further to the above, I ended up spinning up a new ghost droplet on DigitalOcean and migrating content across! All up and running on latest version now, much easier and quicker than trying to resolve the issue with the NodeJS version conflict I had lol