I have used this \pdfstrcmp as in the following to set the different
\makeatletter
\newcommand{\setappendix}{Appendix~\thechapter}
\newcommand{\setchapter}{Chapter~\thechapter~}
\titleformat{\chapter}[display]{\sffamily\huge\bfseries \color{sec}}{}{0.5ex} %
{\ifnum\pdfstrcmp{\@currenvir}{appendices}=0
\setappendix
\else
\setchapter
\fi
\filleft \\%
\titlerule[1pt]
\vspace{1ex}%
#1\filleft\\
\vspace{1ex}%
\titlerule[3pt]}
\makeatother
But I dont know for what reason, though it gives the desire output, it gives me the following popup in texstudio unrecongnized command.

Shall I ignore the popup?
If I dont, will it create any issues?
I am running pdflatex with Miktex 2.9 in Windows 10 machine (and using biber as backend with biblatex).
MWE:
\documentclass[oneside]{book}
\usepackage[explicit]{titlesec}
\usepackage[titletoc]{appendix}
\makeatletter
\newcommand{\setappendix}{Appendix~\thechapter}
\newcommand{\setchapter}{Chapter~\thechapter~}
\titleformat{\chapter}[display]{\sffamily\huge\bfseries}{}{0.5ex} %
{\ifnum\pdfstrcmp{\@currenvir}{appendices}=0
\setappendix
\else
\setchapter
\fi
\filleft \\%
\titlerule[1pt]
\vspace{1ex}%
#1\filleft\\
\vspace{1ex}%
\titlerule[3pt]}
\makeatother
\begin{document}
\chapter{This is usual Chapter}
\begin{appendices}
\chapter{This is appendix}
\end{appendices}
\end{document}
.logfile (as error or slightly less serious as warning). This is just your editor telling you it doesn't know a command. You only need to worry if TeX tells you it doesn't know the command in the.logfile after an attempt to compile your document. – moewe May 10 '18 at 08:48