What happens if you compile the file? Is \fixed just marked red or is there an error when compiling? Older versions of Frescobaldi did not know some commands and treat them as a warning in syntax highlighting even though the installed LilyPond version might understand the syntax.
That being said, a completely red line usually indicates an error reported by LilyPond. So the next question is: What LilyPond version are you using? Cheers, Joram
