Hi Luca -
The first thing to do (if you haven't done it already) would be a full shutdown/restart—if it's just a matter of the OS mistakenly thinking that certain resources are still in use when they'e not (which seems to happen sometimes), then that would hopefully clear it out. You could also re-download the installer, just in case there's a problem there.
If there are older versions of those plug-ins already on your system, you might want to try uninstalling them first with the relevant iZotope uninstaller(s), found in Library > Application Support > iZotope. If none of that works I'd contact iZotope support directly—maybe it's something they've encountered before with their installers..
You didn't mention what OS you're on (it's best to always include that information when troubleshooting, as well as naming the particular products/plug-ins you're having trouble with)—if it's a brand new one like macOS Catalina, note that per their website iZotope's products are not compatible yet with that OS.
Btw, I've moved this thread to the iZotope sub-forum..