writing unicode characters LuaLaTex

Some fonts support U+023A, none that I know supports U+A7CC.

You might be able to get away with U+0338 COMBINING LONG SOLIDUS OVERLAY, but as it's a combining character it should go after the main character, so

S\uni{0338} 

could work. Not with every font, though.

Here I show a comparison using a few font families. When you have decided which one you want, use \setmainfont, of course.

\documentclass[12pt,a4paper]{report} \usepackage[italian]{babel} \usepackage{fontspec} \usepackage{fontsetup} % New Computer Modern % font families for experiments \newfontfamily{\ebgar}{EB Garamond} \newfontfamily{\tmr}{Times New Roman} \newfontfamily{\libser}{Libertinus Serif} \begin{document} \subsubsection*{New Computer Modern} S\symbol{"0338} \symbol{"023A} \subsubsection*{EB Garamond} {\ebgar S\symbol{"0338} \symbol{"023A}} \subsubsection*{Times New Roman} {\tmr S\symbol{"0338} \symbol{"023A}} \subsubsection*{Libertinus} {\libser S\symbol{"0338} \symbol{"023A}} \end{document} 

You see that the result is good only with New Computer Modern and Libertinus Serif. Note also that you don't need \uni, which you can simplify the definition of with

\newcommand{\uni}[1]{\symbol{"#1}} 

so you're able to input \uni{023A}.

If you want to use Times New Roman you can superimpose the solidus U+2044 to an S. It seems to work well also with EB Garamond. Again, I show the comparison with various font families.

\documentclass[12pt,a4paper]{report} \usepackage[italian]{babel} \usepackage{fontspec} \usepackage{fontsetup} % New Computer Modern % font families for experiments \newfontfamily{\ebgar}{EB Garamond} \newfontfamily{\tmr}{Times New Roman} \newfontfamily{\libser}{Libertinus Serif} \newcommand{\Sstroke}{{% \vphantom{\symbol{"2044}S}% for the bounding box \ooalign{S\cr\hidewidth\symbol{"2044}\hidewidth\cr}% }} \begin{document} \subsubsection*{New Computer Modern} \Sstroke{} S\symbol{"0338} \symbol{"023A} \subsubsection*{EB Garamond} {\ebgar \Sstroke{} S\symbol{"0338} \symbol{"023A}} \subsubsection*{Times New Roman} {\tmr \Sstroke{} S\symbol{"0338} \symbol{"023A}} \subsubsection*{Libertinus} {\libser \Sstroke{} S\symbol{"0338} \symbol{"023A}} \end{document} 

CAVEAT

Load neither inputenc nor fontenc along with fontspec. Actually the latter might be loaded in very special situations, which yours isn't.

We can draw the slash over S by \pdfliteral. For example in OpTeX:

\def\slashS{\quitvmode \setbox0=\hbox{S}% \pdfliteral{q \bp{.04em} w 0 \bp{-.3ex} m \bp{\wd0} \bp{1.8ex} l S Q}S} \adef^^^^a7cc{\slashS} \fontfam[lm] Latin Modern: \slashS. \fontfam[Termes] Termes: \slashS. \fontfam[Garamond] EB Garamond: \slashS. \fontfam[Libertinus] Libertinus: \slashS. \bye 

Use the kern command to fine-tune the horizontal positioning between pairs, e.g., S\kern-0.425em/.

MWE

\documentclass[12pt,a4paper]{report} \usepackage{fontspec} \renewcommand{\baselinestretch}{1.5} \setmainfont{EB Garamond} \begin{document} S/ = S -kernXXem /, where XX = \begin{tabular}{ll} S\kern-1.0em{/}&-1.0\\ S\kern-0.9em{/}& -0.9 \\ S\kern-0.8em{/}&-0.8\\ S\kern-0.7em{/}&-0.7\\ S\kern-0.6em{/}&-0.6\\ S\kern-0.5em{/}&-0.5\\ S\kern-0.4em{/}&-0.4\\ S\kern-0.3em{/}&-0.3\\ S\kern-0.2em{/}&-0.2\\ S\kern-0.1em{/}&-0.1\\ S\kern0em{/}&0\\ \end{tabular} \end{document}