Getting the error message “This is not the package you are looking for” when trying to run nvm, why?

see http://nvm.sh

https://github.com/nvm-sh/nvm/blob/master/README.md#install–update-script and run the script that starts with wget:

It should look like this:

wget -qO- https://raw.githubusercontent.com/nvm-sh/nvm/v0.39.1/install.sh | bash

But go copy the script from the link, cause the version might change.
After you do this, close and reopen your terminal.