I think you have the input coordinate format wrong. It needs to be set to MGRS, not DD_2. However, you would need to remove at minimum the space between easting and northing values.
16QCQ 56737 43993 -> 16QCQ 5673743993
However, USNG does allow the space, but data should be in the US, and it assumes NAD83.
Supported Notation Formats (it's Pro doc, but same as ArcGIS Desktop)
Melita