Hey @bryanyuchen ,
Can you start a shell in your Mattermost application container (using portainer if you have that or docker exec -ti <mattermostcontainerid> and try to run a curl in there to the configured TPNS server in your system console?
You can see the configured TPNS server in your System Console → Environment → Push Notification Server
# docker exec -ti <yourcontainerid> curl https://push.mattermost.com
<html><body>Mattermost Push Proxy</body></html>
curling the URL should show the info text, that you successfully reached the push proxy.
But the error message most likely indicates that push notifications are disabled serverside.