Well to be exact, Rebus Farm does not support Beta releases. They will support ForestPack 6 once it is released, I am sure of that.
Until then You have couple of options:
Redo ForestPack objects in Max 2018 and lower with ForestPack 5.4.1
Render with Your own workstations.
Wait for ForestPack 6 official release and Rebus Farm availability. It should be released in coming weeks.