I had this problem turned out that my log file was not being copied to my build directory. The NLog github page had the answer. (I’ve reformatted the paragraph a little for better readability.)
https://github.com/NLog/NLog/wiki/Logging-troubleshooting
NLog cannot find the configuration file. This can happen when the NLog.config file is configured with Build Action = None or Copy to Output Directory = Do not copy in Visual Studio.
Set Build Action = Content and “Copy to Output Directory = Copy if newer to fix this)