nvm is not compatible with the npm config “prefix” option:

$ nvm use v4.2.4

nvm is not compatible with the npm config "prefix" option: 
   currently set to "/Users/z/.npm-global"
Run `npm config delete prefix` or `nvm use --delete-prefix v4.2.4` to unset it.

Delete and Reset the prefix

$ npm config delete prefix 
$ npm config set prefix $NVM_DIR/versions/node/v6.11.1

https://stackoverflow.com/questions/34718528/nvm-is-not-compatible-with-the-npm-config-prefix-option