Is it possible to install and use Flatpak as a non-root user?

[Parke]

Hello,

Is it possible to both install and use Flatpak as a non-root user?  (I
am using Linux 5.3.  I believe user-namespaces are enabled.)

I have (as a non-root user) downloaded, built and installed Flatpak 1.6.0.

I am now trying to install GnuCash.

Flathub says I should run:
flatpak install flathub org.gnucash.GnuCash

(Source: https://flathub.org/apps/details/org.gnucash.GnuCash )

When I run the above command, I get the following error:
Looking for matches?
error: No remote refs found similar to ?flathub?

I found this page:
https://community.clearlinux.org/t/flathub-error-cant-install/2023

Based on the above page, I tried running this command:
flatpak remote-add --if-not-exists flathub
https://dl.flathub.org/repo/flathub.flatpakrepo

I get the following error:
error: The name org.freedesktop.Flatpak.SystemHelper was not provided
by any .service files

My system does not have a desktop environment installed.  I use Xorg
with the Openbox window manager.

Thank you,

Parke