Comment 1 for bug 1899674

Distro packages are not supported here. Distro packages are supported by your distro.

If you cannot reproduce your problem with https://calibre-ebook.com/download_linux then it's not a bug in calibre and you need to ask your distro to fix their intersection of

calibre source code version
python version
qt5 version
various other python module dependencies, or versions thereof

Specifically, the error traceback you reported has been reported several times here, and continues to be not a bug. You're using the 4.99.x unsupported beta version packaged by debian and derivatives (no longer available in Debian, but apparently Ubuntu never got the memo).

tl;dr update to calibre 5.0 or greater, your problems will disappear. Or tell your distro if they really want to package broken betas, they need to sync updated versions of those broken betas from Debian, and do so *promptly*.