@nik3307 @pat000
Here's a few things to run through & double-check:

Check that all Map Editor files are installed correctly:

Example:

Confirm All Map Editor Requirements/Prerequisites are Installed:

Do like @JohnFromGWN says & make sure you have all the requirements/prerequisites installed:

Visual C++ Redistributable for Visual Studio 2013 (install both 'vcredist_x86.exe' & 'vcredist_x64.exe')

Visual C++ Redistributable for Visual Studio 2015 (install both 'vcredist_x86.exe' & 'vcredist_x64.exe')

ScripthookV & ScriptHookVDotNet. Install files to Grand Theft Auto V root folder (where 'GTA5.exe' is):

Example:

NativeUI (This is probably the issue, test using an older version of NativeUI. Start with NativeUI v1.7 (that should work). Once you confirm v1.7 works, you can try newer versions if you want (I can't remember exactly what versions work with Map Editor, but v1.7 certainly does, so try that first ).

NativeUI Version (v1.7, v1.8, v1.9.1 etc) Compatibility Info:
You can usually only have one version of NativeUI installed at a time (certain select mods can have a custom version of NativeUI with a different name that can be installed alongside 'NativeUI.dll', but that's very rare), so generally, if you have mods that explicitly require different versions of NativeUI to work, you'll have to decide before loading the game which version of 'NativeUI.dll' you want installed in your 'scripts' folder.
So, basically, if you have, for example, a mod that explicitly requires NativeUI v1.9.1, it's unlikely that it will function 100% correctly while NativeUI V1.7 is installed. In that case, you would have to manually swap between the different versions of Native UI before loading the game, depending on what you want active for that game session.

Check All Downloaded Files Are Unblocked:

If NativeUI v1.7 doesn't fix it, make sure all of the downloaded ScripthookV/ScriptHookVDotNet/Map Editor files are unblocked in Windows:

Example:

Note:
If you unblock a '.zip/.7zip/.rar' as soon as you download it, that will also automatically unblock every single file within the archive at the same time. Useful to know, as it saves you from unblocking them one by one if there are tons of files in the archive.