Comment by BalinKing
I never realized that “LETTER” in that error referred to paper size—no printer I’ve had has actually given that error, so I only ever heard about it through oblique references to Office Space and such. It makes so much more sense now…
The 'PC' part is paper cassette, it's the printer literally telling you to load letter sized paper into the paper cassette, but everyone acts like it's some mysterious message that's impossible to figure out.