documentclass[a4paper,12pt]{report}
  %------------------------------------------------------------
  usepackage{amssymb,amsmath,amsthm,latexsym,mathrsfs,amsfonts,dsfont}
  %-----accent
 usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{xcolor}
usepackage[colorlinks=true,pagebackref=true]{hyperref}
 hypersetup{urlcolor=blue, citecolor=red, linkcolor=blue}
  newtheorem{corollary}{Corollary}[section]

usepackage[capitalise,noabbrev,nameinlink]{cleveref}
begin{document}
chapter{Preliminary}
 ....The crucial idea of the proof is to make use ..... (see cref{fekete} in cref{app}).

chapter*{Appendix}label{app}
begin{corollary}label{fekete}

end{corollary}
end{document}

documentclass[a4paper,12pt]{report}

%usepackage{amssymb,amsmath,amsthm,latexsym,mathrsfs,amsfonts,dsfont}

usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}

usepackage{xcolor}
usepackage[colorlinks=true,pagebackref=true]{hyperref}

usepackage[capitalise,noabbrev,nameinlink]{cleveref}

hypersetup{urlcolor=blue, citecolor=red, linkcolor=blue}
newtheorem{corollary}{Corollary}[section]


begin{document}

tableofcontents

chapter{Preliminary}
The crucial idea of the proof is to make use ..... (see cref{cor:fekete} in cref{cha:app}).

appendix
chapter{Appendix}label{cha:app} % <=======================================
begin{corollary}label{cor:fekete}

end{corollary}
end{document}