The Max is a different board from the standard model, and its image is not a public download.
Contact Screenly support and we will send you the image and the steps for your device.
We handle it this way because the Max uses Secure Boot, so the device only starts software we signed. Getting the right image onto the right device matters more here than it does on a board without Secure Boot.
Note
As with the standard player, you should not need this to get a new device working. A Player Max ships with ScreenlyOS installed and updates itself once it is online.