There are dozens of linux distributions built on ubunto. They arent ubunto. The small dev team have limited resources and cannot support all the hundreds of linux distributions available.
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There is a known issue with the Steam installer currently; and it's because the Steam build system isn't working for Linux for some reason. We raised a ticket with them 2 weeks ago; but haven't gotten a resolution from Steam yet.
Please use the installer from our web site as a work around until Steam fixes.
Regards,
JPG
On your library, right-click on FG -> Properties -> Beta -> "testforderek - forcing linux build update".
You will be able to download everything afterwards.