Hi @dcarter,
The path ./client/plugins is relative, make sure that the Mattermost binary’s working directory is /opt/mattermost and not /opt/mattermost/bin or another directory.
Try setting the config values to the full path and restart to see if that resolves the issue. If not, see if you’re able to create a file in those directories like this:
$ sudo su mattermost
$ touch /opt/mattermost/plugins/test
$ touch /opt/mattermost/client/plugins/test