Now running as myself:
$ flatpak remote-add --if-not-exists flathub
https://flathub.org/repo/flathub.flatpakrepo
$ flatpak remotes
Name Options
flathub system
$ flatpak install flathub org.frescobaldi.Frescobaldi
Required runtime for org.frescobaldi.Frescobaldi/x86_64/stable
(runtime/org.kde.Platform/x86_64/5.15) found in remote flathub
Do you want to install it? [y/n]: y
Installing in system:
org.kde.Platform/x86_64/5.15 flathub 61f67bf3e852
org.freedesktop.Platform.GL.default/x86_64/20.08 flathub 82a8e234463f
org.freedesktop.Platform.VAAPI.Intel/x86_64/20.08 flathub 842257eb3e76
org.freedesktop.Platform.openh264/x86_64/2.0 flathub 73f998362a6f
org.kde.KStyle.Adwaita/x86_64/5.15 flathub 2f6e4ae8bc10
org.kde.Platform.Locale/x86_64/5.15 flathub 01ece85ac288
org.frescobaldi.Frescobaldi/x86_64/stable flathub 54532dddeb5d
permissions: ipc, network, pulseaudio, x11, devices
file access: home, xdg-config/kdeglobals:ro
dbus access: com.canonical.AppMenu.Registrar
org.frescobaldi.Frescobaldi.Locale/x86_64/stable flathub cc17a220dabb
Is this ok [y/n]: y
Installing: org.kde.Platform/x86_64/5.15 from flathub
error: Failed to install org.kde.Platform/x86_64/5.15:
openat(refs/remotes/flathub/runtime/org.kde.Platform/x86_64/5.15):
Permission denied