Rocket.Chat GCM notifications not showing in LineageOS 16
Typical messaging apps like Rocket.Chat use Firebase Cloud Messaging (FCM formerly known as GCM) for retrieving notifications of new messages from the server.
An Android side FCM required the Google Play Services app.

However LineageOS does not include any Google apps by default. Therefore all apps that use push notifications via Google/FCM will not work correctly. 

If you want to use an app that uses push notification via Google you have to install the Google apps, for example by flashing one of the Open GApps packages - at least the "pico package".

Afterwards you will be able to retrieve push notifications.