KDE is probably too much of a core package to install, unless you are doing for the fun of it. The best way would be to wait until Ubuntu ships it in its next release.
Or reinstall KDE Neon which is Ubuntu base with KDE on top. That's what I've been running as my main desktop at home for 5 years or so and at work on and off for 5 years.
Presumably that would get you the previous release, if running Ubuntu 20.04? You'd either need a custom ppa, build it yourself, or run the early version of 20.10 (oth look at the date, I guess 20.10 is out? So maybe wait for 21.04?).
