I’ve installed Mattermost Team Server 5.22 for Ubuntu 20.04 64bit for the RPi4, and it works well for me, as long as I don’t try to install any Plugins. Here are the installation instructions I followed (and wrote, based on a previous work done by @justinegeffen ):
Why won’t the Plugins work? At present, the Plugins are compiled for AMD64 only (not ARM64). I don’t really need any of the Plugins, currently, so this is an acceptable sacrifice for me, in order to run Mattermost on the Raspberry Pi 4.
Also note, this installs Mattermost from SmartHoneyBee’s unofficial Mattermost build (which seems trustworthy, after talking to the current Mattermost Build maintainer, Harrison Healy). This unofficial build was used, in order to avoid using Docker, which saves about 3GB of RAM (and also avoids the current lack of a MySQL docker image for AMD64). For more details on all these things I’ve just said, please follow the link above.