Make sure your compiled mod folder contains the required Harmony DLLs. Specifically:
CitiesHarmony.API.dll
CitiesHarmony.Harmony.dll

Download Files [drive.google.com]

You can get them from the NuGet packages or your build output. Then copy them to:
C:\SteamLibrary\steamapps\workshop\content\255710\3358543190
or
C:\Users\<your name>\AppData\Local\Colossal Order\Cities_Skylines\Addons\Mods
These files must be in the same folder as your main .dll, like ParallelRoadTool.dll.


Some mods (like Parallel Road Tool) try to write log files to:
Cities_Skylines\Cities_Data\Logs\

But this folder doesn’t exist by default, which causes a crash.
🔨 To fix it:

Navigate to:

C:\SteamLibrary\steamapps\common\Cities_Skylines\Cities_Data\
Create a new folder named:
Logs
Done. No more DirectoryNotFoundException.

You’re good to go. Launch the game, and the mod should work as expected now.
If you're still stuck, check your Harmony version and ensure it's 2.2.2 or higher.