The error indicates that Fiddler cannot load one of its dependencies. Either Fiddler doesn't have access to the required files (which could be caused by a third-party security tool), or the installation of the .NET framework is broken.
Please ensure you've granted full access to Fiddler with any security tools you are using and if that doesn't help, try re-installing the .NET Framework by downloading the latest version supported by your OS: https://dotnet.microsoft.com/en-us/download/dotnet-framework. Let me know if this resolves the issue.