I posted to the Flathub Sigil page about this problem with the inability to access xsel and xclip on September 17. There has been no interest or response on that site. In addition, I get occasional episodes where my Flatpak version of Sigil freezes with the Preview pane greyed out with a message 'Loading'. This never recovers.
Today I've been unable to open and explore a curious ePub originating from a conversion on Google Docs. It never opens sufficiently to allow me explore its curious style entries - always displaying the 'Loading' message in the Preview pane.
So I'm going to try to call it a day with the Flatpak version and attempt to build Sigil myself following the detailed explanation on :
https://github.com/Sigil-Ebook/Sigil...Linux.md#sigil