Each new camera model generally requires explicit support for its raw files in Adobe products and Adobe only updates the current versions of their software with new cameras. Apparently PSE14 stopped getting updates before the 800D was being sold so only PSE15 and newer have support.
You can upgrade to a newer version of Photoshop Elements or use something else like Lightroom and Photoshop via an annual CC Photography subscription (US $120/year) or you can use the free DNG Converter to create DNGs from the CR2s, one folder at a time, and those DNGs but not the CR2s will open in the older Elements. The DNG Converter combines the raw data with the newer camera profile information that the newer Camera Raw plug-ins have about the 800D.
Photoshop Elements: Photoshop Elements | Photoshop.com
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DNG Converter: Adobe software and product updates