! error:  (type 2): cannot find file ''
!  ==> Fatal error occurred, no output PDF file produced!

newcommand{fontmercury}{setmainfont[Path = ./fonts/,
  BoldFont=mercurybd.ttf,
  ItalicFont=mercuryi.ttf,
  BoldItalicFont=mercurybi.ttf
  ]{mercury.ttf}
}

newcommand{fontmercury}{setmainfont[Path = ./assets/fonts/,
  BoldFont=mercurybd.ttf,
  ItalicFont=mercuryi.ttf,
  BoldItalicFont=mercurybi.ttf
  ]{mercury.ttf}
}