documentclass[
  french,
  ]{scrreprt}

usepackage[french]{babel}
usepackage{lastpage}
%usepackage{scrpage2} % Error: chead already defined
usepackage{xwatermark} % needed

pagestyle{fancy}
fancyhf{}
chead{The header}
cfoot{thepage /pageref{LastPage}}
%pagestyle{scrheadings}
%pagestyle{headings} % no headings displayed


begin{document}

FOO

newpage

BAR

end{document}

documentclass[
  french,
  ]{scrreprt}

usepackage[french]{babel}
usepackage{lastpage}
%usepackage{scrpage2} % Error: chead already defined
usepackage{xwatermark} % needed

pagestyle{fancy}
fancyhf{}
renewcommand{headrulewidth}{0pt}
chead{textit{The header}}
cfoot{thepage /pageref{LastPage}}
%pagestyle{scrheadings}
%pagestyle{headings} % no headings displayed


begin{document}

FOO

newpage

BAR

end{document}

renewcommand{headrulewidth}{0pt}

chead{textit{The header}}

documentclass[%
  french,
]{scrreprt}

usepackage[french]{babel}
usepackage{lastpage}

usepackage{xwatermark} % needed, loads fancyhdr :-(( <=================
pagestyle{fancy}
fancyhf{}
chead{textit{The header}} % <=========================================
cfoot{thepage /pageref{LastPage}}
renewcommand{headrulewidth}{0pt} % <==================================


begin{document}

FOO

newpage

BAR

end{document}