Just used the instructions at https://github.com/Sigil-Ebook/Sigil/blob/master/docs/Building_on_Linux.md to build it and it built and installed fine. Linux Mint 20.3
How to Install The Latest Sigil EPUB Ebook Editor 1.6.0: For Ubuntu 18.04 and Linux Mint 19.x, For Ubuntu 21.04, see https://ubuntuhandbook.org/index.php/2021/06/install-sigil-ebook-editor-ubuntu-20-04/ +++ IN MY CASE, Mint 19.3, installation worked fine. +++ Then came an update in the "Software Manager" to 1.7 and ... it does not install on Mint 19.3 any more, many dependencies unresolved.
Verion here uitdated, not as described and newest version (1.6.0) does not install at all. :(
The version in the repository is out of date, 1.3 is now available on the website. Could whoever compiles Sigil for the community add updating the repository version to their long list of priorities :-)