Certain mods that the Medium uses have only been compiled to Windows enabled PCs.
You can still join the medium and interact with other attendees but essential features like panel and stage screens will not work as intended.
You may choose to join in using a virtual machine running Windows or have Windows installed on your machine.
The mods that the Medium uses are only compatible with a specific version of Minecraft. That is Java version 1.12.2.
For how to install an older version of Minecraft, you may click on this link:
Chris Chrome of the development team has created an installer that installs all mods that will be required for the Medium along with the appropriate Minecraft version required to connect. You may find the link to the installer here:
Alternatively you could also get all the mods in a GitHub repository over here