This article details what is causing the error message and how to fix the issue.
When the Farm Scanner Client produces the error:
There will be an entry in the Mainscan.log that is produced in the "Output Folder" (typically C:\Users\<User>\Documents\FarmAuditOutput-* unless changed during configuration of scan). This error will read:
ERROR|Scanner|Method not found: 'Void NLog.Logger.Error(System.Exception, System.String)'.
This issue occurs when there is an installed version of Nlog.dll on Farm in the Global Assembly Cache (GAC) folder, that is out of date. The FarmScanner Client uses the latest version and there is a known issue with NLog.
Download the latest version of NLog.dll and replace the version in the GAC folder.