TIKZ Flow Chart Compile Time Error

Apart from the things mentioned by @Zarko and @CarLateX you may benefit from loading the positioning library and placing the Yes/No nodes differently, and from disclosing the source of your styles.

\documentclass{article} \usepackage[utf8]{inputenc} \usepackage{tikz} \usetikzlibrary{shapes.geometric, arrows,positioning} \begin{document} \tikzset{startstop/.style={rectangle, text width=3cm, minimum height=1cm,text centered, draw=black, fill=red!30}, io/.style={trapezium, trapezium left angle=70, trapezium right angle=110, text width=3cm, minimum height=1cm, text centered, draw=black, fill=blue!30}, process/.style={rectangle, text width=3cm, minimum height=1cm, text centered, draw=black, fill=orange!30}, decision/.style={diamond, text width=3cm, minimum height=1cm, text centered, draw=black, fill=green!30}, arrow/.style={thick,-stealth}} \begin{center} \begin{tikzpicture}[node distance=2cm] \node (start) [startstop] {Start}; \node (in1) [io, right=3cm of start] {Input}; \node (dec1) [decision, below=of start] {Is $t< \epsilon$}; \node (pro1) [process, below=of dec1] {Predict Price Data}; \node (pro2) [process, below=of pro1] {Solve Optimisation Problem}; \node (out1) [io, below=of pro2] {Output $A_i$}; \node (out2) [io, right=of out1] {Output `ERROR'}; \node (dec2) [decision, right of =dec1, xshift=4cm] {Is $t= \epsilon$}; \node (stop) [startstop, below right=2cm and 2cm of out1] {Stop}; \draw [arrow] (start) -- (dec1); \draw [arrow] (in1) -- (dec1); \draw [arrow] (dec1) --(dec2) node[midway,above] {No} ; \draw [arrow] (dec1) -- (pro1) node[midway,left] {Yes}; \draw [arrow] (dec2) -- (out2) node[midway,right] {No}; \draw [arrow] (dec2) -- (stop) node[midway,left] {Yes}; \draw [arrow] (pro1) -- (pro2); \draw [arrow] (pro2) -- (out1); \draw [arrow] (out1.west) -- ++(-1cm,0) |- (dec1); \end{tikzpicture} \end{center} 

mostly off-topic (problem is solved by comment and marmot answer) ... since i never see such a flow chart, let me propose, how i would draw it:

(code is based on my answers on similar questions)

\documentclass{article} \usepackage[utf8]{inputenc} \usepackage{tikz} \usetikzlibrary{arrows.meta, chains, positioning, quotes, shapes.geometric} \tikzset{FlowChart/.style={ base/.style = {draw, minimum height=1cm, minimum width=3cm, text width =\pgfkeysvalueof{/pgf/minimum width}-2*\pgfkeysvalueof{/pgf/inner xsep}, align=center, outer sep=0pt, on chain, join=by arrow}, startstop/.style = {base, fill=red!30}, process/.style = {base, rounded corners, fill=orange!30}, decision/.style = {diamond, aspect=1.3, draw, minimum height=1cm, minimum width=2cm, align=center, on chain, join=by arrow, fill=green!30}, io/.style = {base, trapezium, trapezium stretches body, trapezium left angle=70, trapezium right angle=110, fill=blue!30}, arrow/.style = {semithick, -Triangle} } } \makeatletter \tikzset{suspend join/.code={\def\tikz@after@path{}}} \makeatother \usepackage[active,tightpage]{preview} \PreviewEnvironment{tikzpicture} \setlength\PreviewBorder{1em} \begin{document} \begin{center} \begin{tikzpicture}[FlowChart, node distance = 5mm and 12mm, start chain = going below ] \node (start) [startstop] {Start}; \node (in1) [io] {Input}; \node (dec1) [decision] {Is\\ $t< \epsilon$}; \node (pro1) [process] {Predict Price Data}; \node (pro2) [process] {Solve Optimisation Problem}; \node (out1) [io] {Output $A_i$}; \node (dec2) [decision, suspend join, right=of dec1 -| start.east] {Is\\ $t= \epsilon$}; \node (out2) [io, below=of pro2.south -| dec2] {Output `ERROR'}; \node (stop) [startstop, suspend join, right=of out2] {Stop}; \path (dec1) to ["No"] (pro1); \path (dec2) to ["No"] (out2); \draw [arrow] (dec1) to ["Yes"] (dec2); \draw [arrow] (dec2) to ["Yes"] (dec2 -| stop) -- (stop); % \draw [arrow] (out1.west) -- ++ (-1,0) |- (dec1); % it is illogical ... \end{tikzpicture} \end{center} \end{document} 

compilation report: Errors: 0 Warnings: 0 Bad Boxes: 0