Installing Flatpak on Debian WSL Windows 11

I running Windows 11 and have installed WSL2 and Debian. The instructions for installing flatpak that I am following are at Flatpak—the future of application distribution. The command this page shows is “apt install flatpak”, run as root. The challenge is that this command does not work. When I enter this command Debian returns:
Reading package lists… Done
Building dependency tree
Reading state information… Done
E: Unable to locate package flatpak

Can someone help me with the correct? Have the flatpak folks moved this file elsewhere?

Thanks
Abe

Have you run apt update to refresh the local copy of the package list?

Yes, to no avail. I have even tried to run “apt install flatpak” as root and get the same error message.

Which branch of Debian are you using?

What is a branch? I have no idea how to find that out.