How can I resolve "entry point not found exception" error while importing DLL to c#?

Question

In my code I want to import some methods but still facing error

1. Tested both 32 and 64 bit DLL
2. The DLL is beside the executive file
3. There isn't any dependency for DLL and is unmanaged and standalone

But it still have enry point error *** before all thanks helping me*** error while debugging and importing dll

    [DllImport("sg_api.dll", CallingConvention = CallingConvention.StdCall)]
    public static extern sgStatus sgOpenDevice(ref int device);