Open RAW in Aperture

The Nikon D850 is supported by macOS 10.13 on High Sierra, at least Nikon RAW L format only. But the RAW support is not available in Aperture. None of the RAW Support released since the El Captan can be used with Aperture at all, because Aperture has not been updated for more than two years.

The Digital Camera RAW Compatibility Update 6.17 is the las update that can be used with Aperture. None of the newer updates are recognized by Aperture. Digital camera RAW formats supported by OS X El Capitan - Apple Support

You can use the software that came with your camera to develop the RAWan then import the TIFF files to Aperture, or develop the RAW files in Photos for Mac and then use them in Aperture.

I also am using Adobe DNG converter to convert my D850 NEF files to DNG and then importing without issue into Aperture. The DNG converter transfers most exif info into Aperture. What's missing is 'focus mode' and 'focus distance'. All the rest of the exif data displayed by Aperture is in tact. Other info may be dropped but it is not apparent when I compare my D500 vs D850 exif data in Aperture. ;-)

As Leonie says but you can convert your Nikons RAW files in Adobes DNG converter (free) and then import them to Aperture. Adobe Digital Negative Converter