Running Muscle 5.1 in Windows may result in this error if you are missing a particular Visual C++ redistributable.
Muscle 5.1 requires vcomp140.dll to be present in your C:\Windows\System32directory.
Either install the Visual C++ Distributable: Download Visual C++ Redistributable for Visual Studio 2015 from Official Microsoft Download Center, or otherwise add the vcomp140.dll to your System32 directory to solve this error.