Canon Community

I'm having an issue with my 80D where I get a message on the screen every time I go into live view. The message reads "This function is not available during live view shooting" or "This function is not available in the movie shooting mode" depending ...

Has anyone taken a close look at the image quality and usability(focusing etc) of the Canon 800mm f11 vs the Sigma 150-600's with the 1.4 TC? I own the Sigma and am considering the 800 as a lighter native RF option with an EOS R. thanks in advance