I stumbled upon this one after having reset my mac. Since I've installed ProTools beforehand, the folder @Lindon was talking about was already there, but I was still facing the unable to create directory error.
I used the
chmod 777 Macintosh\ HD/Library/Application\ Support/Avid/Audio/Plug-Ins
command to grant this folder writing permissions and it solved the issue.
I'm leaving this here in case someone stumbles upon this one too.