This can have multiple reasons - can you confirm that the permissions on the filesystem are correct and all files and folders below the /opt/mattermost folder (or whatever your installation path is) is owned by the mattermost user?
You can run the following command to change the permissions:
chown -R mattermost. /opt/mattermost
Try again then, if it still doesn’t work, please check your /opt/mattermost/logs/mattermost.log file for further hints.