I have seen this issue with NPM and yo on Xenial (16.04.3). This isn’t an issue with Mattermost really, but a problem of permissions as outlined here:
https://github.com/yeoman/yeoman.github.io/issues/282
You can try to follow those steps and modify /root/.config /root/.config/configstore but in the end its better to start over with the installation process and follow it to the letter. The most likely cause of this is that you were root at some point in the install and that messed things up.
This comment in particular may shed some light on your issue: https://github.com/yeoman/yeoman.github.io/issues/282#issuecomment-69625203
good luck,
josh