documentclass[a4paper, oneside, 12pt, parskip=half]{scrbook}

usepackage[a4paper,
  vmargin=2cm, hmargin=2cm, % page margins
  includehead, includefoot, % Margins calculated include header and footer
  footskip=2em]
{geometry}

usepackage[footsepline=0.25pt, headsepline=0.25pt, automark]{scrlayer-scrpage}

% automark places chapter/section title in header. Also enables placement in footer.
automark[section]{chapter}

usepackage{lastpage}

ohead{rightmark}

% Footer content
cfoot{tiny{Page thepage  of  pageref*{LastPage} \
leftmark}}

usepackage{lipsum}

begin{document}

chapter{Chap One}

lipsum[1]

section{Sec One.One}

lipsum[2-5]

section{Sec One.Two}

lipsum[6-10]

chapter{Chap Two}

lipsum[11-15]

end{document}