We recommend against using nvm in production because you can end up with different OS users having different versions of node installed without realising it (this is why we added the warning). It’s recommended to uninstall nvm and install node directly system-wide https://ghost.org/docs/install/ubuntu/#install-nodejs
The reason running with node 14 causes errors is because the CLI is finding Ghost running a different version. In other words, the CLI node version is not the same as the Ghost node version, which is why Kevin said using something like nvm (or n) is not suggested