specflow.exe is compiled for .NET 3.5, it cannot load .NET 4.0 assemblies by default. To generate this report for .NET 4.0 projects, you have to force specflow.exe to use the .NET 4.0 runtime by using the config file.
You need to create one config file called specflow.exe.config with the following content:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<startup>
<supportedRuntime version="v4.0.30319" />
</startup>
</configuration>
save and copy to same folder where specflow.exe resides.
Command line should be : specFlow.exe generateAll "C:\Project\MyProject.csproj" /force /verbose
hope this helps.