@Errorflin Try rolling back to 4.8, there might be incompatibilities.

@Errorflin very strange, are you sure you selected the correct project template, c# dotnet framework 4.8 and not some other template?

@nikolysor you're replacing streamed models with normal/non streamed? Is it just the textures you're changing?

@nikolysor link?

@Errorflin If you don't have them your OS is unlikely to be working or you didn't install VS properly.
Add reference, Assemblies, type system.window in the searchbox and it absolutely has to appear. If it doesn't you have a major issues!

@Errorflin I'm sorry which assemblies did you download. Windows.Forms does not need to be downloaded.
Make sure none of "Using" are greyed out or underlined in red.

The New to Modding Checklist.

1. What should I have installed for modding GTA5 Single Player?
The two following files are the absolute essentials if you are going to add mods. Do not download from other links to ensure you have the current versions.

Script Hook V
Open IV

2. Where do I install Script Hook V?
Script Hook V does not have an installer, you simply copy the 2 files below (from the extracted package) to the same folder as your GTA5.exe. You can call this your game folder, or installation folder, or root folder.

When you unzip or extract the downloaded Script Hook package you will see a bin folder. That's the only folder you want. There are 3 files in the bin folder of the Script Hook V download. You only need two of them: ScriptHookV.dll and dinput8.dll. Both copied in the game folder. You don't copy the bin folder, just those 2 files. You don't need NativeTrainer.asi.

3. Where do I install Open IV?
Open IV, pronounced Open Four, has an installer. Just accept the default path for the installation and follow the instructions. Open IV will ask you if you want to install three different files. Say yes to all of them. They are: ASI Loader, OpenIV.asi, and openCamera. ASI loader is exactly the same as dinput8.dll from ScriptHook V but that's ok. openCamera will be useful if you use the R* editor. OpenIV.asi is a must.

4. How do I know my files are properly installed?
If you followed the instructions carefully for OpenIV then the program itself will be installed in a hidden windows folder called AppData. You will be able to see OpenIV.asi, ScriptHookV.dll and dinput8.dll in the same folder as GTA5.exe. Like this:

@John_CK ok im super confused. You wrote you didn't have any mods yet you installed open4 and scripthook? And when you say scripthook is giving an error message, you mean a gray message box before game even starts to load?

1. Right click GTA5.exe and from the properties what is the version number.
2. Please do the same for scripthook which should be installed in the same folder as GTA5.exe

Openiv should be in the same folder as well.

Let's take a step back?
What ide or compiler are you using? Visual Studio and C#?
Are you using dot netframework 4.8 or higher?
Did you get any compiler errors or warnings or just at runtime?
What assemblies are you referencing, scripthookvdotnet 3, windows, external libraries. Sounds like you're referencing something incompatible or missing.
You don’t need to put a scripthook log, not relevant.
And what tutorial are you referencing?

If you want post your code or at least your references in pastebin.

@SlippyBippy Scripthook is routinely updated and fully compatible with the most recent update of the game.