P: Fails to import photos and stalls out on macOS 10.15 Catalina

Onyx for Catalina is out: https://www.titanium-software.fr/en/onyx.html 

Actually, bitch at Apple.

Now, perhaps a solution. On your MAC bring up the System Preferences. Select the Security an Privacy icon. Select the Privacy tab. Scroll down to Files and Folders. Make sure Adobe Lightroom Classic and while you are at it a Photoshop have full rights.

I agree totally. It's very important that Adobe or Apple, solve this problem as soon as possible.

to clarify, multiple error messages, one of which is directly useful, and none of which are returned until I manually stop the hung/stillborn import process

OK. reading again your email, I found workaround.
Instead of importing pics from my camera, I imported them from SD reader, and proceeded as you suggested. It finally worked.
Nice bug 🙂
Hope Adobe (or Apple ?) will fix it soon
Thx
Laurent

Another one failed due to lack of disk space, but it just hung and only returned an error message after I stopped the import. The general problem seems to be that import does not detect a failure. Error messages could be clearer as well.

Inspect/replace card reader?

I'm having same issue with imports from SD cards to new iMac - files don't complete - shows exclamation mark and error. Thought it was a corrupted card, but works fine on older MacBook Pro running an older version of LR CC. Both computers have been upgraded to Catalina (bad decision) but laptop is working, and the iMac is not.

I had the same issue, import never starts, error messages about a few things, but one was that the destination folder was not writable. My issue was that the import presets had changed on upgrade from LR5, and it had indeed defaulted to the wrong folder. Trivial to fix, but egregious error handling.

Jean you should blame your IT guys for upgrading you too soon, not Adobe (not this time!)