nvEncodeAPI64.dll is missing

Hi,
I am trying build NvEncoder on Win10 Visual Studio 2015/17 NVIDIA driver 376.51
however I am getting error code from
nvStatus = nvEncodeAPICreateInstance(m_pEncodeAPI) = 15

After some investigation it seems that nvEncodeAPI64.dll is missing.

… any known resolutions for the problem?

Regards
MichaelT