[2020] "Could not locate a valid installation of the Paradox Launcher"

After uninstalling the launcher, make sure to check that no files are left behind. You will want to look for and delete these folders as well (if they exist):

C:\Users\\AppData\Roaming\Paradox Interactive\launcher-v2
C:\Users\\AppData\Local\Paradox Interactive\launcherpath (should be gone)
C:\Users\\AppData\Local\Paradox Interactive\launcher-v2 (should only contain logs)

Then start the game through Steam and the launcher will install itself again. If you're still having issues after doing that, then I really recommend contacting Paradox Support, so they can help you get things running and log any bugs that might need fixing.