Installing
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Then Add Flatpak Ubuntu Mate Linux PPA.
sudo add-apt-repository ppa:alexlarsson/flatpak
sudo apt update
Authenticate with the User Admin Pass.
If Got “User is Not in Sudoers file” then see: How to Enable sudo. - 
Finally, Install Flatpak.
sudo apt install flatpak
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How to Getting Started Searching & Installing Flatpak Apps
 
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