I've been facing this problem that as far as i could see, i wasn't the only one that had to deal with it. To be fair i'm sure most of you guys could find any fix, but personally i had to figure it out myself what the problem was, and maybe someone in future experiences the same issue and this fix might be helpful (i hope so).
For starters, check your "ScriptHookDotNet.log" file that's in GTA IV's root folder. Go to bottom, exactly to the point were the script stack was written.
If you are getting a prompt saying something similar to "System.PlatformNotSupportedException: No voice installed on the system or none available with the current security setting." then you keep reading this article. So the problem here is that your system doesn't have any Text-Speech voice installed, something that shouldn't happen though. I believe that when you install Windows it should come with a voice installed in your system already, but it doesn't matter, nothing you cannot fix.
In order to fix this, -proceed to open Settings.
-once there, go to Time & Language
-go to Voice
-scroll all the way down until you find an option to add voices. Downwards you should have a list containing all the installed voice packages.
it should be empty, in case it is not, then check if the package English USA it's not installed.
If it's not installed, install it by clicking "Add Voices", search English and select the USA package.
Wait until the package has finished installing. Hop into your game and try to see if the problem was shooted away. I didn't have to restart my PC by the way,
but do it in case you believe it would be necessary.
I'll attach pictures step by step in case you missed something:
Thanks for reading, hope you could get the mod working!