Lets say to want to make default version as 10.19.0.
nvm alias default v10.19.0
But it will give following error
! WARNING: Version 'v10.19.0' does not exist.
default -> v10.19.0 (-> N/A)
In That case you need to run two commands in the following order
# Install the version that you would like
nvm install 10.19.0
# Set 10.19.0 (or another version) as default
nvm alias default 10.19.0