There are three common causes for this error.
1. The Mi-Enterprise Middleware installer turns on some Windows features that typically require a reboot to take effect. If you have not done so since running the installer, reboot your server or VM.
2. There may be a higher-up configuration file disallowing some of the Windows features required by Mi-Enterprise Middleware. Check for other web config files in the folders above the folder containing the Mi-Enterprise Middleware config file.
3. The application configuration may be incorrect. Use the IIS manager to ensure that the MFS application is associated with the MFS application pool and is configured to run on .NET framework version 4.0. Then go into the application's advanced settings and ensure that it has 32-bit applications enabled.
If none of the above steps solve your issue, contact Mi-Co Support.