Hey @Taiki and welcome to the Mattermost forums!
The omnibus setup comes with SSL enabled by default as it uses Let’s encrypt. Could it be that there has been an error with the Let’s encrypt certificate generation during the setup or did you maybe answer the question if you want to have Let’s encrypt with no?
When you change settings in /etc/mattermost/mmomni.mattermost.env you need to restart Mattermost, so that it recognizes the new environment variables, but please do not touch this file for now and instead please run mmomni reconfigure and post the output here.