High sierra 10.13.6 i cannot open Canon R6 raws CR3

High Sierra does not support the R6,

See the list: Digital camera RAW formats supported by iOS 11 and macOS High Sierra - Apple Support

So, you need to upgrade macOS if possible or use an app such as Adobe DNG Converter to convert them to .dng files.

Adobe DNG Converter for Mac

App requirements: 

• Intel 64
• Mac OS X 10.3.9 or later

License: 

Free • Absolutely Free

Download Adobe DNG Converter for Mac | MacUpdate

Please note the latest version of Adobe DNG converter requires a mac to be running macOS Mojave 10.14

or later.

https://helpx.adobe.com/camera-raw/using/adobe-dng-converter.html

Ah, thanks, MacUpdate wasn't very open on Sys Reqs!