ASC is unable to do any installs on my iMac (macOS 10.14.4), because the OS is repeatedly asking for permission to install some helper app, and every install simply fails. I am quite upset! I need my V Collection NOW!!!
Calm down, don't have a heart attack over it. Here's what's worked for me:
1) Install ASC, log in, then quit.
2) download the VSTs one by one from the Arturia downloads page. The downloads will save in your downloads folder.
3) manually install the VSTs one by one from the downloads folder.
4) after you've installed them all, re-open the ASC, sign back in and wait (probably for ages) while it authenticates your instruments.
Installing the 20-odd VSTs manually is a time-consuming ballache I admit, and that's why there's a helper tool which is supposed to do it all in the background. But I'm thinking the most recent helper tool in the ASC update a few days ago hasn't been signed properly, because you're not the first person to complain that it's failing MacOS security protocols. Strange thing is it seems to be affecting only some computers not others.
Good luck.