first of all, have you chosen the right connection on the back of the AVR? not all of them support 4k.
possibly test with another source. possibly a setting on the tv? some tv devices allow settings per connection.
or at denon - Video | Output Settings - Video Conversion - "Resolution (HDMI)" -> Auto.
Hi,
4K passthrough is the passing through of picture information in Ultra HD resolution. The error message could be caused either by the HDMI cable or the TV. For both, perhaps check whether it is HDCP 2.2 or HDMI 2 capable. That would be my approach ;)
Greetings