Canon 800D raw unsupported image format Sierra 10.12.6

The RAW Support for your Canon Camera came out with macOS 10.12.5 Sierra. Unfortunately, Aperture cannot use any of the new RAW support released with macOS Sierra. Aperture is no longer developed, and the last compatibility update has been for Yosemite OS X 10.10.3. If you want to keep using Aperture for your new camera, you will have to develop the RAW files in a different application and then convert them to TIFF or another format, that Aperture can read. Photos for Mac can develop the RAW files and export TIFF, or use Adobe's DNG converter.

Did you import the photos to Photos before you installed the Sierra Update?

In that case you may need to reprocess the RAW files before Photos can recognize them. Open one of the RAW files in Edit mode and try to use the command "Image > Reprocess RAW" if it is available.