Are you finding mssetup.dll just press download button to get your mssetup.dll file.
If some software in your windows OS wants mssetup.dll your operating system will ask for DLL file. When that DLL file is not available you will get this error.
If you have moved your file or changed the name of dll file you will get this error.
You will mostly see there errors when you dll file is missing:
If you need mssetup.dll Download it right now and install it into your system to fix the error as soon as possible. just Install mssetup.dll and fix your error.