Thank you so much @Skywheel. After a long search I found your post. Accordingly, installation of Joplin desktop on Raspberry Pi 4 with Ubuntu 21.04 worked flawlessly:

1.Install flatpak

sudo apt-get install flatpak

  1. Add repository

flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo

3.Install Joplin

flatpak install flathub net.cozic.joplin_desktop

On first impression, everything (sync, webclipper) is working.

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