Getting a Roblox Ubuntu installation to work is difficult. The game hasn't been ported to Linux, and so for a long time, the only way to get it working was through the Windows environment Wine. This is no longer an option, but you can get Roblox Studio to work through Wine or use a purpose-built management application such as Grapejuice. Unfortunately, other options such as Roblox Lutris no longer work either, so your options are limited.
Only Roblox Studio Works
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The bad news is that Roblox Player no longer works on Ubuntu. It used to work through Wine, but now it is picked up by Roblox's anti-cheat code, which forces the game to quit. Nobody has identified a workaround for the Roblox Player, so if you're using Ubuntu or a Linux system, you can only use Roblox Studio. As a result, you can create and test games within the Roblox environment, but you can't participate other than that. You won't be able to play games created by other players.
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Installing Studio Directly
The simplest approach to installing Roblox Studio is to do it directly through the terminal, although the alternative (see next section) is to use a wrapper program that takes away a lot of the work in terms of maintenance. However, for installation, this direct approach has advantages because it only requires a handful of commands, and then you can download Studio and start creating.
Here is the list of commands:
sudo dpkg --add-architecture i386
wget -nc https://dl.winehq.org/wine-builds/winehq.key
sudo apt-key add winehq.key
sudo add-apt-repository 'deb https://dl.winehq.org/wine-builds/ubuntu/ focal main'
sudo apt update
sudo apt install --install-recommends winehq-stable
winecfg
Enter these commands one at a time, allowing the processes to complete between each. Essentially this process installs Wine and configures it correctly so you can use Roblox Studio.
Finally, use your browser and download Roblox Studio at www.roblox.com/Create. Then you can install it through the Wine environment.
Roblox Linux Download Simplified
This alternative approach uses a management program called Grapejuice. There are a few lines of commands to enter to install this properly, but it saves much of the work in terms of configuration. The first command is the same as the previous approach:
sudo dpkg --add-architecture i386
Then run these two commands to make sure all the repositories are up to date:
sudo apt update
sudo apt upgrade -y
Then use this long command to install the necessary libraries:
sudo apt install -y wine wine64 wine32 git python3-pip pkg-config libcairo2-dev libdbus-1-dev gtk-update-icon-cache desktop-file-utils xdg-utils libgirepository1.0-dev gir1.2-gtk-3.0
Get the source code for the program with this command:
git clone https://gitlab.com/brinkervii/grapejuice.git
Finally, when this is complete, install Grapejuice with these two lines:
cd grapejuice
python3 ./install.py
Then, run the application and install Roblox Studio by going to the "Maintenance" tab and choosing "Install Roblox." When this is complete, go to the "Launcher" tab and run Roblox from there.